Quarterly report pursuant to Section 13 or 15(d)

Unsecured Line of Credit and Term Notes - Additional Information (Detail)

v3.10.0.1
Unsecured Line of Credit and Term Notes - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Oct. 30, 2018
Dec. 07, 2017
Jul. 21, 2016
Jun. 20, 2016
Apr. 08, 2014
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2011
Debt Instrument [Line Items]                      
Term notes, outstanding amount           $ 1,625,000,000   $ 1,625,000,000   $ 1,625,000,000  
Unamortized senior term note discount           4,539,000   4,539,000   $ 4,949,000  
Amortization expense related to deferred debt issuance costs               2,672,000 $ 2,496,000    
Maximum [Member]                      
Debt Instrument [Line Items]                      
Leverage ratio percentage       60.00%              
Indenture covenant percentage on unencumbered assets       150.00%              
Minimum [Member]                      
Debt Instrument [Line Items]                      
Interest coverage ratio on debt       150.00%              
Secured Debt [Member] | Maximum [Member]                      
Debt Instrument [Line Items]                      
Leverage ratio percentage       40.00%              
Revolving Credit Facility [Member]                      
Debt Instrument [Line Items]                      
Revolving credit limit           $ 500,000,000   $ 500,000,000      
Basis spread over LIBOR               1.10%      
Facility fee               0.15%      
Interest rate, line of credit facility           3.28%   3.28%   2.63%  
Amount available on unsecured revolving line of credit           $ 371,200,000   $ 371,200,000      
Line of credit facility, expiration date               Dec. 10, 2019      
Revolving Credit Facility [Member] | Subsequent Event [Member]                      
Debt Instrument [Line Items]                      
Revolving credit limit $ 500,000,000                    
Basis spread over LIBOR 0.95%                    
Facility fee 0.15%                    
Line of credit facility, expiration date Mar. 10, 2023                    
Maturity in April 8, 2024 [Member]                      
Debt Instrument [Line Items]                      
Debt instrument,maturity date         Apr. 08, 2024            
Debt instrument stated interest rate         4.533%            
Additional secured term note         $ 175,000,000            
Maturity in April 8, 2024 [Member] | Maximum [Member]                      
Debt Instrument [Line Items]                      
Debt instrument stated interest rate         6.283%            
Senior Term Note Due July 1, 2026 [Member] | Unsecured Senior Notes [Member]                      
Debt Instrument [Line Items]                      
Debt instrument,maturity date       Jul. 01, 2026              
Debt instrument principal amount       $ 600,000,000   600,000,000   $ 600,000,000      
Debt instrument stated interest rate       3.50%              
Debt instrument percentage discount to par       0.553%              
Unamortized senior term note discount       $ 3,300,000              
Underwriting discount and other offering expenses       5,500,000              
Proceeds from senior notes, net       $ 591,200,000              
Term Note Due July 21, 2028 [Member]                      
Debt Instrument [Line Items]                      
Debt instrument,maturity date     Jul. 21, 2028                
Term notes, outstanding amount           200,000,000   200,000,000   $ 200,000,000  
Debt instrument stated interest rate     3.67%                
Additional secured term note     $ 200,000,000                
Senior Notes and Term Notes [Member]                      
Debt Instrument [Line Items]                      
Amortization expense related to deferred debt issuance costs           500,000 $ 500,000 $ 1,600,000 $ 1,600,000    
Term Note Due June 4, 2020 [Member]                      
Debt Instrument [Line Items]                      
Basis spread over LIBOR               1.15%      
Debt instrument,maturity date               Jun. 04, 2020      
Debt instrument principal amount           325,000,000   $ 325,000,000      
Repayments of unsecured term note                   225,000,000  
Term notes, outstanding amount           $ 100,000,000   $ 100,000,000   $ 100,000,000  
Debt instrument variable interest rate           3.25%   3.25%   2.53%  
Term Note Due June 4, 2020 [Member] | Revolving Credit Facility [Member] | Subsequent Event [Member]                      
Debt Instrument [Line Items]                      
Basis spread over LIBOR 1.00%                    
Debt instrument,maturity date Jun. 04, 2020                    
Debt instrument principal amount $ 100,000,000                    
Term Note | Revolving Credit Facility [Member] | Maximum [Member] | Subsequent Event [Member]                      
Debt Instrument [Line Items]                      
Debt instrument principal amount $ 900,000,000                    
Senior Term Note Due December 15, 2027 [Member] | Unsecured Senior Notes [Member]                      
Debt Instrument [Line Items]                      
Debt instrument,maturity date   Dec. 15, 2027                  
Debt instrument principal amount   $ 450,000,000                  
Debt instrument stated interest rate   3.875%                  
Debt instrument,first required payment   Jun. 15, 2018                  
Debt instrument percentage discount to par   0.477%                  
Unamortized senior term note discount   $ 2,100,000                  
Underwriting discount and other offering expenses   4,000,000                  
Proceeds from senior notes, net   $ 443,900,000                  
Term Note Due August 5, 2021 [Member]                      
Debt Instrument [Line Items]                      
Debt instrument,maturity date                     Aug. 05, 2021
Term notes, outstanding amount           $ 100,000,000   $ 100,000,000   $ 100,000,000  
Debt instrument stated interest rate                     5.54%
Additional secured term note                     $ 100,000,000
Term Note Due August 5, 2021 [Member] | Maximum [Member]                      
Debt Instrument [Line Items]                      
Debt instrument stated interest rate                     7.29%