Investment in Joint Ventures - Summary of Unconsolidated Joint Ventures' Financial Statements (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Investment in storage facilities, net	$ 3,675,667		$ 3,675,667		$ 3,697,096
Other assets	4,487		4,487		4,863
Total Assets	3,862,969		3,862,969		3,876,774
Mortgages payable	12,489		12,489		12,674
Total Liabilities	1,828,885		1,828,885		1,829,078
Unaffiliated partners’ equity	0		0		0
Company equity	2,013,440		2,013,440		2,028,323
Total Equity	2,013,440		2,013,440		$ 2,028,323
Depreciation and amortization of customer list	(24,784)	$ (38,105)	(49,548)	$ (75,747)
Amortization of financing fees			(1,781)	(1,663)
Income tax expense	(700)	2,500	(1,200)	2,200
Interest expense	(17,518)	$ (15,716)	(34,722)	$ (30,926)
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Investment in storage facilities, net	1,076,193		1,076,193
Investment in office building, net	4,712		4,712
Other assets	20,821		20,821
Total Assets	1,101,726		1,101,726
Due to the Company	994		994
Mortgages payable	468,606		468,606
Other liabilities	8,290		8,290
Total Liabilities	477,890		477,890
Unaffiliated partners’ equity	491,003		491,003
Company equity	132,833		132,833
Total Equity	623,836		623,836
Total Liabilities and Partners’ Equity	$ 1,101,726		1,101,726
Total revenues			55,627
Property operating expenses			(17,295)
Administrative, management and call center fees			(4,314)
Depreciation and amortization of customer list			(12,822)
Amortization of financing fees			(653)
Income tax expense			(155)
Interest expense			(8,716)
Net income			$ 11,672