Segment Information - Schedule of Financial Information (Detail) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues:	$ 337,505			$ 306,953			$ 972,653	$ 889,251
Operating expenses:	174,867			153,362			503,545	457,032
Income from operations	162,638			184,398			470,313	463,026
Management fees and other income	13,000			10,120			36,063	30,849
General and administrative expense	(22,519)			(19,707)			(68,548)	(62,822)
Depreciation and amortization expense	(56,051)			(52,283)			(165,116)	(155,924)
Gain on real estate transactions	0			30,807			1,205	30,807
Interest expense	(46,908)			(45,926)			(141,716)	(130,239)
Non-cash interest expense related to amortization of discount on equity component of exchangeable senior notes	(1,186)			(1,140)			(3,533)	(3,525)
Interest income	2,799			1,371			5,905	3,997
Equity in earnings of unconsolidated real estate ventures	2,704			3,622			8,455	10,648
Income tax expense	(4,052)			(2,638)			(8,580)	(6,077)
Net income	115,995	$ 112,689	$ 102,160	139,687	$ 102,713	$ 95,430	330,844	337,830
Operating Segments
Segment Reporting Information [Line Items]
Revenues:	324,505			296,833			936,590	858,402
Operating expenses:	96,297			81,372			269,881	238,286
Income from operations	228,208			215,461			666,709	620,116
Operating Segments | Self-Storage Operations
Segment Reporting Information [Line Items]
Revenues:	290,917			266,728			841,504	772,742
Operating expenses:	88,653			73,652			248,288	219,488
Income from operations	202,264			193,076			593,216	553,254
Operating Segments | Tenant Reinsurance
Segment Reporting Information [Line Items]
Revenues:	33,588			30,105			95,086	85,660
Operating expenses:	7,644			7,720			21,593	18,798
Income from operations	$ 25,944			$ 22,385			$ 73,493	$ 66,862