Segment Information - Schedule of Financial Information (Detail) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenues:	$ 343,024			$ 337,505			$ 1,002,722	$ 972,653
Operating expenses:	179,817			174,867			532,331	503,545
Net operating income:	163,207			162,638			470,391	470,313
Management fees and other income	13,307			13,000			38,299	36,063
General and administrative expense	(23,894)			(22,519)			(72,242)	(68,548)
Depreciation and amortization expense	(56,412)			(56,051)			(167,705)	(165,116)
Gain on real estate transactions	0			0			0	1,205
Interest expense	(42,213)			(46,908)			(127,610)	(141,716)
Non-cash interest expense related to amortization of discount on equity component of exchangeable senior notes	(1,233)			(1,186)			(3,675)	(3,533)
Interest income	3,145			2,799			6,488	5,905
Equity in earnings and dividend income from unconsolidated real estate entities	5,605			2,704			15,692	8,455
Income tax expense	(4,657)			(4,052)			(10,013)	(8,580)
Net income	123,854	$ 111,256	$ 116,162	115,995	$ 112,689	$ 102,160	351,273	330,844
Operating Segments
Segment Reporting Information [Line Items]
Revenues:	329,717			324,505			964,423	936,590
Operating expenses:	99,511			96,297			292,384	269,881
Net operating income:	230,206			228,208			672,039	666,709
Self-Storage Operations | Operating Segments
Segment Reporting Information [Line Items]
Revenues:	290,423			290,917			856,438	841,504
Operating expenses:	92,322			88,653			271,659	248,288
Net operating income:	198,101			202,264			584,779	593,216
Tenant Reinsurance | Operating Segments
Segment Reporting Information [Line Items]
Revenues:	39,294			33,588			107,985	95,086
Operating expenses:	7,189			7,644			20,725	21,593
Net operating income:	$ 32,105			$ 25,944			$ 87,260	$ 73,493