Annual report pursuant to Section 13 and 15(d)

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

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Investment in Storage Facilities and Intangible Assets - Schedule of Purchase Price of Facilities Acquired (Detail)
12 Months Ended
Dec. 31, 2018
USD ($)
Property
Dec. 31, 2017
USD ($)
Property
Dec. 31, 2016
USD ($)
Investment Holdings [Line Items]      
Number of Properties | Property 8 2  
Purchase Price $ 77,738,000 $ 22,638,000  
Consideration paid, Cash Paid 72,601,000 22,626,000  
Consideration paid, Value of Operating Partnership Units Issued 3,547,000   $ 9,500,000
Consideration paid, Mortgage Assumed 1,392,000   11,300,000
Consideration paid, Net Other Liabilities Assumed (Assets Acquired) 198,000 12,000  
Acquisition Date Fair Value, In-Place Customer Leases 1,156,000    
Closing Costs Expensed 0 0 $ 29,542,000
Land [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets 17,317,000 1,881,000  
Building, Equipment and Improvements [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets $ 59,265,000 $ 20,757,000  
NH [Member] | Asset Acquired on 9/4/2018 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property 1    
Date of Acquisition Sep. 04, 2018    
Purchase Price $ 5,641,000    
Consideration paid, Cash Paid 5,609,000    
Consideration paid, Net Other Liabilities Assumed (Assets Acquired) 32,000    
Acquisition Date Fair Value, In-Place Customer Leases 108,000    
NH [Member] | Asset Acquired on 9/4/2018 [Member] | Land [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets 1,257,000    
NH [Member] | Asset Acquired on 9/4/2018 [Member] | Building, Equipment and Improvements [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets $ 4,276,000    
CA [Member] | Asset Acquired on 9/18/2018 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property 1    
Date of Acquisition Sep. 18, 2018    
Purchase Price $ 13,846,000    
Consideration paid, Cash Paid 13,800,000    
Consideration paid, Net Other Liabilities Assumed (Assets Acquired) 46,000    
Acquisition Date Fair Value, In-Place Customer Leases 206,000    
CA [Member] | Asset Acquired on 9/18/2018 [Member] | Land [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets 2,089,000    
CA [Member] | Asset Acquired on 9/18/2018 [Member] | Building, Equipment and Improvements [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets $ 11,551,000    
CA [Member] | Asset Acquired on 12/7/2018 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property 1    
Date of Acquisition Dec. 07, 2018    
Purchase Price $ 9,547,000    
Consideration paid, Cash Paid 9,524,000    
Consideration paid, Net Other Liabilities Assumed (Assets Acquired) 23,000    
Acquisition Date Fair Value, In-Place Customer Leases 85,000    
CA [Member] | Asset Acquired on 12/7/2018 [Member] | Land [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets 1,331,000    
CA [Member] | Asset Acquired on 12/7/2018 [Member] | Building, Equipment and Improvements [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets $ 8,131,000    
NY [Member] | Asset Acquired on 10/2/2018 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property 1    
Date of Acquisition Oct. 02, 2018    
Purchase Price $ 8,124,000    
Consideration paid, Cash Paid 8,118,000    
Consideration paid, Net Other Liabilities Assumed (Assets Acquired) 6,000    
Acquisition Date Fair Value, In-Place Customer Leases 231,000    
NY [Member] | Asset Acquired on 10/2/2018 [Member] | Land [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets 3,357,000    
NY [Member] | Asset Acquired on 10/2/2018 [Member] | Building, Equipment and Improvements [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets $ 4,536,000    
NY [Member] | Asset Acquired on 12/20/2018 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property 1    
Date of Acquisition Dec. 20, 2018    
Purchase Price $ 7,264,000    
Consideration paid, Cash Paid 2,267,000    
Consideration paid, Value of Operating Partnership Units Issued 3,547,000    
Consideration paid, Mortgage Assumed 1,392,000    
Consideration paid, Net Other Liabilities Assumed (Assets Acquired) 58,000    
Acquisition Date Fair Value, In-Place Customer Leases 156,000    
NY [Member] | Asset Acquired on 12/20/2018 [Member] | Land [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets 3,970,000    
NY [Member] | Asset Acquired on 12/20/2018 [Member] | Building, Equipment and Improvements [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets $ 3,138,000    
GA [Member] | Asset Acquired on 11/1/2018 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property 1    
Date of Acquisition Nov. 01, 2018    
Purchase Price $ 14,234,000    
Consideration paid, Cash Paid 14,241,000    
Consideration paid, Net Other Liabilities Assumed (Assets Acquired) (7,000)    
Acquisition Date Fair Value, In-Place Customer Leases 89,000    
GA [Member] | Asset Acquired on 11/1/2018 [Member] | Land [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets 1,666,000    
GA [Member] | Asset Acquired on 11/1/2018 [Member] | Building, Equipment and Improvements [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets $ 12,479,000    
FL [Member] | Asset Acquired on 12/11/2018 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property 1    
Date of Acquisition Dec. 11, 2018    
Purchase Price $ 9,781,000    
Consideration paid, Cash Paid 9,751,000    
Consideration paid, Net Other Liabilities Assumed (Assets Acquired) 30,000    
Acquisition Date Fair Value, In-Place Customer Leases 233,000    
FL [Member] | Asset Acquired on 12/11/2018 [Member] | Land [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets 2,014,000    
FL [Member] | Asset Acquired on 12/11/2018 [Member] | Building, Equipment and Improvements [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets $ 7,534,000    
MO [Member] | Asset Acquired on 12/27/2018 [Member]      
Investment Holdings [Line Items]      
Number of Properties | Property 1    
Date of Acquisition Dec. 27, 2018    
Purchase Price $ 9,301,000    
Consideration paid, Cash Paid 9,291,000    
Consideration paid, Net Other Liabilities Assumed (Assets Acquired) 10,000    
Acquisition Date Fair Value, In-Place Customer Leases 48,000    
MO [Member] | Asset Acquired on 12/27/2018 [Member] | Land [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets 1,633,000    
MO [Member] | Asset Acquired on 12/27/2018 [Member] | Building, Equipment and Improvements [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets $ 7,620,000    
IL [Member] | Asset Acquired on 2/23/2017 [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  
IL [Member] | Asset Acquired on 2/23/2017 [Member] | Land [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets   771,000  
IL [Member] | Asset Acquired on 2/23/2017 [Member] | Building, Equipment and Improvements [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets   $ 9,318,000  
NC [Member] | Asset Acquired on 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)  
NC [Member] | Asset Acquired on 12/14/2017 [Member] | Land [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets   1,110,000  
NC [Member] | Asset Acquired on 12/14/2017 [Member] | Building, Equipment and Improvements [Member]      
Investment Holdings [Line Items]      
Acquisition Date Fair Value, Fixed Assets   $ 11,439,000