Investment in Joint Ventures - Summary of Unconsolidated Joint Ventures' Financial Statements (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Investment in storage facilities, net	$ 3,681,129		$ 3,697,096
Other assets	4,603		4,863
Total Assets	3,868,914		3,876,774
Mortgages payable	12,581		12,674
Total Liabilities	1,831,731		1,829,078
Unaffiliated partners’ equity	0		0
Company equity	2,019,337		2,028,323
Total Equity	2,019,337		$ 2,028,323
Depreciation and amortization of customer list	(24,764)	$ (37,642)
Amortization of financing fees	(891)	(832)
Interest expense	(17,204)	$ (15,210)
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Investment in storage facilities, net	1,074,836
Investment in office building, net	4,761
Other assets	14,172
Total Assets	1,093,769
Due to the Company	900
Mortgages payable	463,930
Other liabilities	6,835
Total Liabilities	471,665
Unaffiliated partners’ equity	489,409
Company equity	132,695
Total Equity	622,104
Total Liabilities and Partners’ Equity	1,093,769
Total revenues	27,377
Property operating expenses	(8,677)
Administrative, management and call center fees	(2,135)
Depreciation and amortization of customer list	(6,467)
Amortization of financing fees	(374)
Income tax expense	(77)
Interest expense	(4,121)
Net income	$ 5,526