Segment Information - Schedule of Financial Information of Business Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Operating expenses:	$ 153,362	$ 144,275	$ 457,032	$ 422,676
Income from operations	153,591	139,881	432,219	400,491
General and administrative expense	(19,707)	(19,498)	(62,822)	(60,171)
Gain (loss) on real estate transactions and impairment of real estate	30,807	0	30,807	(6,019)
Interest expense	(45,926)	(39,766)	(130,239)	(113,192)
Non-cash interest expense related to amortization of discount on equity component of exchangeable senior notes	(1,140)	(1,268)	(3,525)	(3,827)
Interest income	1,371	1,401	3,997	5,201
Equity in earnings of unconsolidated real estate ventures	3,622	3,990	10,648	11,407
Income tax expense	(2,638)	(3,163)	(6,077)	(9,154)
Net income	139,687	101,075	337,830	284,907
Operating Segments
Segment Reporting Information [Line Items]
Revenues:	296,833	274,471	858,402	793,928
Operating expenses:	81,372	76,702	238,286	218,366
Income from operations	215,461	197,769	620,116	575,562
Property management fees and other income	10,120	9,685	30,849	29,239
General and administrative expense	(19,707)	(19,498)	(62,822)	(60,171)
Depreciation and amortization expense	(52,283)	(48,075)	(155,924)	(144,139)
Gain (loss) on real estate transactions and impairment of real estate	30,807	0	30,807	(6,019)
Interest expense	(45,926)	(39,766)	(130,239)	(113,192)
Non-cash interest expense related to amortization of discount on equity component of exchangeable senior notes	(1,140)	(1,268)	(3,525)	(3,827)
Interest income	1,371	1,401	3,997	5,201
Equity in earnings of unconsolidated real estate ventures	3,622	3,990	10,648	11,407
Income tax expense	(2,638)	(3,163)	(6,077)	(9,154)
Net income	139,687	101,075	337,830	284,907
Operating Segments | Self-Storage Operations
Segment Reporting Information [Line Items]
Revenues:	266,728	248,589	772,742	720,878
Operating expenses:	73,652	70,430	219,488	204,370
Income from operations	193,076	178,159	553,254	516,508
Operating Segments | Tenant Reinsurance
Segment Reporting Information [Line Items]
Revenues:	30,105	25,882	85,660	73,050
Operating expenses:	7,720	6,272	18,798	13,996
Income from operations	$ 22,385	$ 19,610	$ 66,862	$ 59,054