Investment in Joint Ventures - Summary of Unconsolidated Joint Ventures' Financial Statements (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Investment in storage facilities, net	$ 3,958,024		$ 3,958,024				$ 3,694,258
Other assets	26,200		26,200				5,921
Total Assets	4,188,740		4,188,740				3,892,212
Mortgages payable	35,016		35,016				12,302
Total Liabilities	2,027,398		2,027,398				1,810,759
Unaffiliated partners’ equity	0		0				0
Company equity	2,135,318	$ 2,009,561	2,135,318	$ 2,009,561	$ 2,041,920	$ 2,046,891	2,057,737	$ 2,013,440	$ 2,019,337	$ 2,028,323
Total Equity	2,135,318		2,135,318				$ 2,057,737
Depreciation and amortization of customer list	(26,854)	(27,291)	(79,239)	(76,839)
Amortization of financing fees			(2,873)	(2,672)
Interest expense	(19,760)	$ (17,923)	(56,339)	$ (52,645)
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Investment in storage facilities, net	1,244,834		1,244,834
Investment in office building, net	4,514		4,514
Other assets	23,931		23,931
Total Assets	1,273,279		1,273,279
Due to the Company	1,124		1,124
Mortgages payable	563,255		563,255
Other liabilities	14,336		14,336
Total Liabilities	578,715		578,715
Unaffiliated partners’ equity	544,792		544,792
Company equity	149,772		149,772
Total Equity	694,564		694,564
Total Liabilities and Partners’ Equity	$ 1,273,279		1,273,279
Total revenues			96,058
Property operating expenses			(29,681)
Administrative, management and call center fees			(7,518)
Depreciation and amortization of customer list			(20,650)
Amortization of financing fees			(662)
Income tax expense			(237)
Interest expense			(16,939)
Net income			$ 20,371