Investment in Joint Ventures - Summary of Unconsolidated Joint Ventures' Financial Statements (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Investment in storage facilities, net	$ 4,357,660		$ 4,357,660				$ 3,993,140
Other assets	33,221		33,221				29,591
Total Assets	4,689,055		4,689,055				4,232,964
Mortgages payable	34,352		34,352				34,851
Total Liabilities	2,413,319		2,413,319				2,073,763
Unaffiliated partners’ equity	0		0				0
Company equity	2,249,888	$ 2,135,318	2,249,888	$ 2,135,318	$ 2,132,681	$ 2,147,719	2,132,894	$ 2,041,920	$ 2,046,891	$ 2,057,737
Total Equity	2,249,888		2,249,888				2,132,894
Total Liabilities and Shareholders' Equity/ Partners' Capital	4,689,055		4,689,055				$ 4,232,964
Total revenues	156,310	145,634	450,266	427,186
Depreciation and amortization of customer list	(33,018)	(26,854)	(90,335)	(79,239)
Amortization of financing fees			(3,033)	(2,873)
Interest expense	(20,544)	(19,760)	(61,056)	(56,339)
Net income	37,288	$ 140,746	110,560	$ 216,346
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Investment in storage facilities, net	1,083,191		1,083,191
Investment in office building, net	4,109		4,109
Other assets	68,834		68,834
Total Assets	1,156,134		1,156,134
Due to the Company	3,223		3,223
Mortgages payable	525,713		525,713
Other liabilities	24,761		24,761
Total Liabilities	553,697		553,697
Unaffiliated partners’ equity	459,105		459,105
Company equity	143,332		143,332
Total Equity	602,437		602,437
Total Liabilities and Shareholders' Equity/ Partners' Capital	$ 1,156,134		1,156,134
Total revenues			99,902
Property operating expenses			(31,102)
Administrative, management and call center fees			(5,547)
Gain on disposal of self-storage facilities			217,607
Depreciation and amortization of customer list			(23,383)
Amortization of financing fees			(590)
Income tax expense			(235)
Interest expense			(17,638)
Net income			$ 239,014