Segment Information - Schedule of Financial Information of Business Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 1,356,212	$ 1,308,454	$ 1,196,604
Operating expenses	708,147	674,701	607,708
Net operating income	666,140	634,958	619,703
Property management fees and other income	52,129	49,890	41,757
General and administrative expense	(96,594)	(89,418)	(81,256)
Depreciation and amortization expense	(224,444)	(219,857)	(209,050)
Gain on real estate transactions	18,075	1,205	30,807
Interest expense	(168,626)	(186,526)	(178,436)
Non-cash interest expense related to amortization of discount on equity component of exchangeable senior notes	(3,675)	(4,742)	(4,687)
Interest income	15,192	7,467	5,292
Equity in earnings and dividend income from unconsolidated real estate entities	22,361	11,274	14,452
Income tax expense	(13,810)	(11,308)	(9,244)
Net income	517,582	451,123	447,080
Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,304,083	1,258,564	1,154,847
Operating expenses	387,109	365,426	317,402
Net operating income	916,974	893,138	837,445
Operating Segments | Self-Storage Operations
Segment Reporting Information [Line Items]
Revenues	1,157,522	1,130,177	1,039,340
Operating expenses	360,615	336,050	291,695
Net operating income	796,907	794,127	747,645
Operating Segments | Tenant Reinsurance
Segment Reporting Information [Line Items]
Revenues	146,561	128,387	115,507
Operating expenses	26,494	29,376	25,707
Net operating income	$ 120,067	$ 99,011	$ 89,800