Segment Information - Schedule of Financial Information of Business Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Total revenues	$ 296,813	$ 276,003	$ 582,298	$ 539,011
Operating expenses, including depreciation and amortization	152,097	139,596	303,670	278,401
Income from operations	144,716	136,407	278,628	260,610
General and administrative expense	(21,651)	(21,865)	(43,115)	(40,673)
Loss on real estate transactions and impairment of real estate	0	(6,019)	0	(6,019)
Interest expense	(43,347)	(37,456)	(84,313)	(73,426)
Non-cash interest expense related to amortization of discount on equity component of exchangeable senior notes	(1,176)	(1,290)	(2,385)	(2,559)
Interest income	1,188	1,485	2,626	3,800
Equity in earnings of unconsolidated real estate ventures	3,429	3,838	7,026	7,417
Income tax (expense) benefit	(2,097)	(2,867)	(3,439)	(5,991)
Net income	102,713	94,098	198,143	183,832
Operating Segments
Segment Reporting Information [Line Items]
Total revenues	286,649	265,109	561,569	519,457
Operating expenses, including depreciation and amortization	78,554	71,099	156,914	141,664
Income from operations	208,095	194,010	404,655	377,793
Property management fees and other income	10,164	10,894	20,729	19,554
General and administrative expense	(21,651)	(21,865)	(43,115)	(40,673)
Depreciation and amortization expense	(51,892)	(46,632)	(103,641)	(96,064)
Loss on real estate transactions and impairment of real estate	0	(6,019)	0	(6,019)
Interest expense	(43,347)	(37,456)	(84,313)	(73,426)
Non-cash interest expense related to amortization of discount on equity component of exchangeable senior notes	(1,176)	(1,290)	(2,385)	(2,559)
Interest income	1,188	1,485	2,626	3,800
Equity in earnings of unconsolidated real estate ventures	3,429	3,838	7,026	7,417
Income tax (expense) benefit	(2,097)	(2,867)	(3,439)	(5,991)
Net income	102,713	94,098	198,143	183,832
Operating Segments | Self-storage operations
Segment Reporting Information [Line Items]
Total revenues	258,128	240,796	506,014	472,289
Operating expenses, including depreciation and amortization	73,083	67,295	145,836	133,940
Income from operations	185,045	173,501	360,178	338,349
Operating Segments | Tenant reinsurance
Segment Reporting Information [Line Items]
Total revenues	28,521	24,313	55,555	47,168
Operating expenses, including depreciation and amortization	5,471	3,804	11,078	7,724
Income from operations	$ 23,050	$ 20,509	$ 44,477	$ 39,444