Segment Information - Schedule of Financial Information (Detail) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenues:	$ 412,492			$ 343,024			$ 1,149,979	$ 1,002,722
Operating expenses:	186,214			179,817			549,682	532,331
Net operating income:	226,278			163,207			664,180	470,391
Management fees and other income	16,879			13,307			47,320	38,299
General and administrative expense	(24,395)			(23,894)			(74,276)	(72,242)
Depreciation and amortization expense	(61,516)			(56,412)			(179,685)	(167,705)
Gain on real estate transactions	0			0			63,883	0
Interest expense	(39,670)			(42,213)			(120,605)	(127,610)
Non-cash interest expense related to amortization of discount on equity component of exchangeable senior notes	0			(1,233)			0	(3,675)
Interest income	11,729			3,145			36,871	6,488
Equity in earnings and dividend income from unconsolidated real estate entities	8,255			5,605			23,533	15,692
Equity in earnings of unconsolidated real estate ventures - gain on sale of real estate assets and purchase of joint venture partner's interest	0			0			6,251	0
Income tax expense	(6,772)			(4,657)			(16,330)	(10,013)
Net income	199,820	$ 178,579	$ 215,501	123,854	$ 111,257	$ 116,162	593,900	351,273
Operating Segments
Segment Reporting Information [Line Items]
Revenues:	395,613			329,717			1,102,659	964,423
Operating expenses:	100,303			99,511			295,721	292,384
Net operating income:	295,310			230,206			806,938	672,039
Self-Storage Operations | Operating Segments
Segment Reporting Information [Line Items]
Revenues:	351,355			290,423			976,448	856,438
Operating expenses:	92,794			92,322			274,316	271,659
Net operating income:	258,561			198,101			702,132	584,779
Tenant Reinsurance | Operating Segments
Segment Reporting Information [Line Items]
Revenues:	44,258			39,294			126,211	107,985
Operating expenses:	7,509			7,189			21,405	20,725
Net operating income:	$ 36,749			$ 32,105			$ 104,806	$ 87,260