Segment Information - Schedule of Financial Information (Detail) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenues:	$ 323,602		$ 296,813		$ 635,148	$ 582,298
Operating expenses:	165,609		152,097		328,678	303,670
Income from operations	159,198		144,716		307,675	278,628
Management fees and other income	12,317		10,164		23,063	20,729
General and administrative expense	(23,351)		(21,651)		(46,029)	(43,115)
Depreciation and amortization expense	(54,406)		(51,892)		(109,065)	(103,641)
Gain on real estate transactions	1,205		0		1,205	0
Interest expense	(47,448)		(43,347)		(94,808)	(84,313)
Non-cash interest expense related to amortization of discount on equity component of exchangeable senior notes	(1,185)		(1,176)		(2,347)	(2,385)
Interest income	1,718		1,188		3,106	2,626
Equity in earnings of unconsolidated real estate ventures	3,121		3,429		5,751	7,026
Income tax expense	(2,715)		(2,097)		(4,528)	(3,439)
Net income	112,689	$ 102,160	102,713	$ 95,430	214,849	198,143
Operating Segments
Segment Reporting Information [Line Items]
Revenues:	311,285		286,649		612,085	561,569
Operating expenses:	87,852		78,554		173,584	156,914
Income from operations	223,433		208,095		438,501	404,655
Operating Segments | Self-Storage Operations
Segment Reporting Information [Line Items]
Revenues:	279,584		258,128		550,587	506,014
Operating expenses:	80,870		73,083		159,635	145,836
Income from operations	198,714		185,045		390,952	360,178
Operating Segments | Tenant Reinsurance
Segment Reporting Information [Line Items]
Revenues:	31,701		28,521		61,498	55,555
Operating expenses:	6,982		5,471		13,949	11,078
Income from operations	$ 24,719		$ 23,050		$ 47,549	$ 44,477