Annual report pursuant to Section 13 and 15(d)

Investment in Storage Facilities - Schedule of Acquired Facilities and Purchase Price of Facilities (Detail)

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Investment in Storage Facilities - Schedule of Acquired Facilities and Purchase Price of Facilities (Detail)
12 Months Ended
Dec. 31, 2017
USD ($)
Property
Dec. 31, 2016
USD ($)
Property
Dec. 31, 2015
USD ($)
Investment Holdings [Line Items]      
Number of Properties | Property 2 122  
Purchase Price $ 22,638,000    
Consideration paid, Cash Paid 22,626,000 $ 1,796,900,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired) 12,000    
Acquisition Date Fair Value, Land 1,881,000    
Acquisition Date Fair Value, Building, Equipment, and Improvements 20,757,000    
Acquisition costs $ 0 $ 29,542,000 $ 2,991,000
Acquisitions 2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   122  
Purchase Price   $ 1,783,920,000  
Consideration paid, Cash Paid   1,796,923,000  
Consideration paid, Value of Operating Partnership Units Issued   9,516,000  
Consideration paid, Mortgage Assumed   11,251,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   (33,770,000)  
Acquisition Date Fair Value, Land   310,428,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   1,403,601,000  
Acquisition Date Fair Value, In-Place Customers Leases   53,391,000  
Acquisition Date Fair Value, Trade Name   16,500,000  
Acquisition costs   $ 29,887,000  
Illinois [Member] | Date Of Acquisition, 2/23/17 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property 1    
Date of Acquisition Feb. 23, 2017    
Purchase Price $ 10,089,000    
Consideration paid, Cash Paid 10,076,000    
Consideration paid, Net Other Liabilities Assumed (Assets Acquired) 13,000    
Acquisition Date Fair Value, Land 771,000    
Acquisition Date Fair Value, Building, Equipment, and Improvements $ 9,318,000    
Illinois [Member] | Date Of Acquisition, 11/17/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   1  
Date of Acquisition   Nov. 17, 2016  
Purchase Price   $ 8,884,000  
Consideration paid, Cash Paid   7,125,000  
Consideration paid, Value of Operating Partnership Units Issued   1,750,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   9,000  
Acquisition Date Fair Value, Land   371,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   $ 8,513,000  
NC [Member] | Date Of Acquisition, 12/14/2017 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property 1    
Date of Acquisition Dec. 14, 2017    
Purchase Price $ 12,549,000    
Consideration paid, Cash Paid 12,550,000    
Consideration paid, Net Other Liabilities Assumed (Assets Acquired) (1,000)    
Acquisition Date Fair Value, Land 1,110,000    
Acquisition Date Fair Value, Building, Equipment, and Improvements $ 11,439,000    
Florida [Member] | Date Of Acquisition, 1/6/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   4  
Date of Acquisition   Jan. 06, 2016  
Purchase Price   $ 20,350,000  
Consideration paid, Cash Paid   20,246,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   104,000  
Acquisition Date Fair Value, Land   6,646,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   13,339,000  
Acquisition Date Fair Value, In-Place Customers Leases   365,000  
Acquisition costs   $ 437,000  
Florida [Member] | Date Of Acquisition, 2/12/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   1  
Date of Acquisition   Feb. 12, 2016  
Purchase Price   $ 11,274,000  
Consideration paid, Cash Paid   11,270,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   4,000  
Acquisition Date Fair Value, Land   2,294,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   $ 8,980,000  
Florida [Member] | Date Of Acquisition, 5/2/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   1  
Date of Acquisition   May 02, 2016  
Purchase Price   $ 8,100,000  
Consideration paid, Cash Paid   4,006,000  
Consideration paid, Mortgage Assumed   4,036,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   58,000  
Acquisition Date Fair Value, Land   3,018,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   4,922,000  
Acquisition Date Fair Value, In-Place Customers Leases   160,000  
Acquisition costs   $ 161,000  
Florida [Member] | Date Of Acquisition, 9/27/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   1  
Date of Acquisition   Sep. 27, 2016  
Purchase Price   $ 10,500,000  
Consideration paid, Cash Paid   10,407,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   93,000  
Acquisition Date Fair Value, Land   2,809,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   7,523,000  
Acquisition Date Fair Value, In-Place Customers Leases   168,000  
Acquisition costs   $ 244,000  
Florida [Member] | Date Of Acquisition, 12/20/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   1  
Date of Acquisition   Dec. 20, 2016  
Purchase Price   $ 9,800,000  
Consideration paid, Cash Paid   6,900,000  
Consideration paid, Mortgage Assumed   2,966,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   (66,000)  
Acquisition Date Fair Value, Land   3,268,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   6,378,000  
Acquisition Date Fair Value, In-Place Customers Leases   154,000  
Acquisition costs   $ 98,000  
CA [Member] | Date Of Acquisition, 1/21/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   4  
Date of Acquisition   Jan. 21, 2016  
Purchase Price   $ 80,603,000  
Consideration paid, Cash Paid   80,415,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   188,000  
Acquisition Date Fair Value, Land   28,420,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   51,145,000  
Acquisition Date Fair Value, In-Place Customers Leases   1,038,000  
Acquisition costs   $ 397,000  
CA [Member] | Date Of Acquisition, 3/16/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   3  
Date of Acquisition   Mar. 16, 2016  
Purchase Price   $ 68,832,000  
Consideration paid, Cash Paid   63,965,000  
Consideration paid, Value of Operating Partnership Units Issued   4,472,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   395,000  
Acquisition Date Fair Value, Land   22,647,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   45,371,000  
Acquisition Date Fair Value, In-Place Customers Leases   814,000  
Acquisition costs   $ 313,000  
CA [Member] | Date Of Acquisition, 3/17/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   1  
Date of Acquisition   Mar. 17, 2016  
Purchase Price   $ 17,320,000  
Consideration paid, Cash Paid   17,278,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   42,000  
Acquisition Date Fair Value, Land   6,728,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   10,339,000  
Acquisition Date Fair Value, In-Place Customers Leases   253,000  
Acquisition costs   $ 132,000  
CA [Member] | Date Of Acquisition, 4/11/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   1  
Date of Acquisition   Apr. 11, 2016  
Purchase Price   $ 36,750,000  
Consideration paid, Cash Paid   33,346,000  
Consideration paid, Value of Operating Partnership Units Issued   3,294,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   110,000  
Acquisition Date Fair Value, Land   17,445,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   18,840,000  
Acquisition Date Fair Value, In-Place Customers Leases   465,000  
Acquisition costs   $ 141,000  
NH [Member] | Date Of Acquisition, 1/21/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   5  
Date of Acquisition   Jan. 21, 2016  
Purchase Price   $ 55,435,000  
Consideration paid, Cash Paid   55,151,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   284,000  
Acquisition Date Fair Value, Land   13,281,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   41,237,000  
Acquisition Date Fair Value, In-Place Customers Leases   917,000  
Acquisition costs   $ 657,000  
Massachusetts [Member] | Date Of Acquisition, 1/21/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   1  
Date of Acquisition   Jan. 21, 2016  
Purchase Price   $ 11,387,000  
Consideration paid, Cash Paid   11,362,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   25,000  
Acquisition Date Fair Value, Land   4,880,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   6,341,000  
Acquisition Date Fair Value, In-Place Customers Leases   166,000  
Acquisition costs   $ 81,000  
Texas [Member] | Date Of Acquisition, 1/21/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   3  
Date of Acquisition   Jan. 21, 2016  
Purchase Price   $ 38,975,000  
Consideration paid, Cash Paid   38,819,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   156,000  
Acquisition Date Fair Value, Land   19,796,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   18,598,000  
Acquisition Date Fair Value, In-Place Customers Leases   581,000  
Acquisition costs   $ 299,000  
Texas [Member] | Date Of Acquisition, 5/5/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   1  
Date of Acquisition   May 05, 2016  
Purchase Price   $ 10,800,000  
Consideration paid, Cash Paid   10,708,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   92,000  
Acquisition Date Fair Value, Land   2,333,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   8,302,000  
Acquisition Date Fair Value, In-Place Customers Leases   165,000  
Acquisition costs   $ 133,000  
Arizona [Member] | Date Of Acquisition, 2/1/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   1  
Date of Acquisition   Feb. 01, 2016  
Purchase Price   $ 9,275,000  
Consideration paid, Cash Paid   9,261,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   14,000  
Acquisition Date Fair Value, Land   988,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   8,224,000  
Acquisition Date Fair Value, In-Place Customers Leases   63,000  
Acquisition costs   $ 136,000  
Pennsylvania [Member] | Date Of Acquisition, 2/17/2016[Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   1  
Date of Acquisition   Feb. 17, 2016  
Purchase Price   $ 5,750,000  
Consideration paid, Cash Paid   5,732,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   18,000  
Acquisition Date Fair Value, Land   1,768,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   3,879,000  
Acquisition Date Fair Value, In-Place Customers Leases   103,000  
Acquisition costs   $ 164,000  
CO [Member] | Date Of Acquisition, 2/29/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   1  
Date of Acquisition   Feb. 29, 2016  
Purchase Price   $ 12,600,000  
Consideration paid, Cash Paid   12,549,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   51,000  
Acquisition Date Fair Value, Land   4,528,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   7,915,000  
Acquisition Date Fair Value, In-Place Customers Leases   157,000  
Acquisition costs   $ 188,000  
CO [Member] | Date Of Acquisition, 8/4/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   1  
Date of Acquisition   Aug. 04, 2016  
Purchase Price   $ 8,900,000  
Consideration paid, Cash Paid   8,831,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   69,000  
Acquisition Date Fair Value, Land   5,062,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   3,679,000  
Acquisition Date Fair Value, In-Place Customers Leases   159,000  
Acquisition costs   $ 119,000  
Connecticut [Member] | Date Of Acquisition, 4/14/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   2  
Date of Acquisition   Apr. 14, 2016  
Purchase Price   $ 17,313,000  
Consideration paid, Cash Paid   17,152,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   161,000  
Acquisition Date Fair Value, Land   6,142,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   10,904,000  
Acquisition Date Fair Value, In-Place Customers Leases   267,000  
Acquisition costs   $ 204,000  
New York [Member] | Date Of Acquisition, 4/26/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   2  
Date of Acquisition   Apr. 26, 2016  
Purchase Price   $ 24,312,000  
Consideration paid, Cash Paid   20,143,000  
Consideration paid, Mortgage Assumed   4,249,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   (80,000)  
Acquisition Date Fair Value, Land   5,710,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   18,201,000  
Acquisition Date Fair Value, In-Place Customers Leases   401,000  
Acquisition costs   $ 372,000  
New York [Member] | Date Of Acquisition, 5/19/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   2  
Date of Acquisition   May 19, 2016  
Purchase Price   $ 8,400,000  
Consideration paid, Cash Paid   8,366,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   34,000  
Acquisition Date Fair Value, Land   714,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   7,521,000  
Acquisition Date Fair Value, In-Place Customers Leases   165,000  
Acquisition costs   $ 213,000  
Other States [Member] | Date Of Acquisition, 7/15/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   83  
Date of Acquisition   Jul. 15, 2016  
Purchase Price   $ 1,299,740,000  
Consideration paid, Cash Paid   1,335,274,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   (35,534,000)  
Acquisition Date Fair Value, Land   150,660,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   1,085,750,000  
Acquisition Date Fair Value, In-Place Customers Leases   46,830,000  
Acquisition Date Fair Value, Trade Name   16,500,000  
Acquisition costs   $ 25,398,000  
South Carolina [Member] | Date Of Acquisition, 7/29/2016 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property   1  
Date of Acquisition   Jul. 29, 2016  
Purchase Price   $ 8,620,000  
Consideration paid, Cash Paid   8,617,000  
Consideration paid, Net Other Liabilities Assumed (Assets Acquired)   3,000  
Acquisition Date Fair Value, Land   920,000  
Acquisition Date Fair Value, Building, Equipment, and Improvements   $ 7,700,000