Segment Information - Schedule of Financial Information of Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 307,353	$ 306,953	$ 296,813	$ 285,485	$ 281,842	$ 284,156	$ 276,003	$ 263,008	$ 1,196,604	$ 1,105,009	$ 991,875
Operating expenses	150,676	153,362	152,097	151,573	140,728	144,275	139,596	138,805	607,708	563,404	542,037
Net operating income	156,677	153,591	144,716	133,912	141,114	139,881	136,407	124,203	619,703	654,394	458,303
Property management fees and other income									41,757	39,379	39,842
General and administrative expense									(81,256)	(78,961)	(81,806)
Gain on real estate transactions, earnout from prior acquisitions and impairment of real estate									30,807	112,789	8,465
Interest expense									(178,436)	(153,511)	(133,479)
Non-cash interest expense related to amortization of discount on equity component of exchangeable senior notes									(4,687)	(5,103)	(4,980)
Interest income									5,292	6,736	10,998
Equity in earnings of unconsolidated real estate ventures									14,452	15,331	12,895
Equity in earnings of unconsolidated real estate ventures - gain on sale of real estate assets and purchase of partners' interests									0	0	69,199
Income tax expense									(9,244)	(3,625)	(15,847)
Net income	$ 109,250	$ 139,687	$ 102,713	$ 95,430	$ 229,315	$ 101,075	$ 94,098	$ 89,734	447,080	514,222	397,089
Operating Segments
Segment Reporting Information [Line Items]
Revenues									1,154,847	1,065,630	952,033
Operating expenses									317,402	291,147	265,560
Net operating income									837,445	774,483	686,473
Operating Segments | Self-Storage Operations
Segment Reporting Information [Line Items]
Revenues									1,039,340	967,229	864,742
Operating expenses									291,695	271,974	250,005
Net operating income									747,645	695,255	614,737
Operating Segments | Tenant Reinsurance
Segment Reporting Information [Line Items]
Revenues									115,507	98,401	87,291
Operating expenses									25,707	19,173	15,555
Net operating income									89,800	79,228	71,736
Segment reconciling items
Segment Reporting Information [Line Items]
Property management fees and other income									41,757	39,379	39,842
General and administrative expense									(81,256)	(78,961)	(81,806)
Depreciation and amortization expense									(209,050)	(193,296)	(182,560)
Acquisition and other related costs									0	0	(12,111)
Gain on real estate transactions, earnout from prior acquisitions and impairment of real estate									30,807	112,789	8,465
Interest expense									(178,436)	(153,511)	(133,479)
Non-cash interest expense related to amortization of discount on equity component of exchangeable senior notes									(4,687)	(5,103)	(4,980)
Interest income									5,292	6,736	10,998
Equity in earnings of unconsolidated real estate ventures									14,452	15,331	12,895
Equity in earnings of unconsolidated real estate ventures - gain on sale of real estate assets and purchase of partners' interests									0	0	69,199
Income tax expense									$ (9,244)	$ (3,625)	$ (15,847)