Investment in Joint Ventures - Summary of Unconsolidated Joint Ventures' Financial Statements (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Investment in storage facilities, net	$ 4,457,145				$ 3,993,140				$ 4,457,145	$ 3,993,140
Other assets	31,907				29,591				31,907	29,591
Total Assets	4,727,848				4,232,964				4,727,848	4,232,964
Mortgages payable	37,777				34,851				37,777	34,851
Total Liabilities	2,323,304				2,073,763				2,323,304	2,073,763
Unaffiliated partners’ equity	0				0				0	0
Company equity	2,378,098				2,132,894				2,378,098	2,132,894	$ 2,057,737	$ 2,028,323
Total Equity	2,378,098				2,132,894				2,378,098	2,132,894
Total Liabilities and Shareholders' Equity/ Partners' Capital	4,727,848				4,232,964				4,727,848	4,232,964
Total revenues	166,505	$ 156,310	$ 147,013	$ 146,943	147,555	$ 145,634	$ 145,028	$ 136,522	616,771	574,739	550,850
Depreciation and amortization of customer list									(122,925)	(107,130)	(102,530)
Amortization of financing fees									(4,096)	(3,900)	(3,621)
Interest expense									(86,015)	(76,430)	(70,672)
Net income	41,799	$ 37,288	$ 36,648	$ 36,625	$ 43,730	$ 140,746	$ 40,964	$ 34,637	152,360	$ 260,077	$ 207,558
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Investment in storage facilities, net	1,084,776								1,084,776
Investment in office building, net	4,374								4,374
Other assets	61,045								61,045
Total Assets	1,150,195								1,150,195
Due to the Company	1,064								1,064
Mortgages payable	533,361								533,361
Other liabilities	17,089								17,089
Total Liabilities	551,514								551,514
Unaffiliated partners’ equity	458,159								458,159
Company equity	140,522								140,522
Total Equity	598,681								598,681
Total Liabilities and Shareholders' Equity/ Partners' Capital	$ 1,150,195								1,150,195
Total revenues									126,847
Property operating expenses									(39,318)
Administrative, management and call center fees									(9,590)
Gain on sale of self-storage facilities									219,694
Depreciation and amortization of customer list									(29,575)
Amortization of financing fees									(962)
Income tax expense									(239)
Interest expense									(22,704)
Net income									$ 244,153