Investment in Joint Ventures - Summary of Company's Unconsolidated Joint Ventures (Parenthetical) (Detail) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) Property	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Property
Schedule of Equity Method Investments [Line Items]
Number of storage facilities acquired | Property	0
Mortgages payable	$ 12,489		$ 12,674
Distributions from unconsolidated joint ventures	4,103	$ 2,852
Sovran HHF Storage Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Contribution of company to joint venture as share in capital	200
Distributions from unconsolidated joint ventures	2,700
Excess of investment over net asset due to capitalization of acquisition related costs	1,700
Sovran HHF Storage Holdings II LLC [Member]
Schedule of Equity Method Investments [Line Items]
Distributions from unconsolidated joint ventures	1,000
191 III Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Contribution of company to joint venture as share in capital	100
Distributions from unconsolidated joint ventures	200
Iskalo Office Holdings, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Distributions from unconsolidated joint ventures	100
Rent paid during the period	600	$ 600
Urban Box Coralway Storage, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Contribution of company to joint venture as share in capital	300
SNL/Orix 1200 McDonald Ave., LLC [Member]
Schedule of Equity Method Investments [Line Items]
Non-recourse mortgage loan principal outstanding	8,600
SNL Orix Merrick, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Non-recourse mortgage loan principal outstanding	11,300
Review Avenue Partners LLC [Member] | Common Stock Shares [Member]
Schedule of Equity Method Investments [Line Items]
Contribution of company to joint venture as share in capital	200
NYX Don Mills Storage LP [Member] | Common Stock Shares [Member]
Schedule of Equity Method Investments [Line Items]
Contribution of company to joint venture as share in capital	$ 1,000
Arizona [Member] | Sovran HHF Storage Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	11
Colorado [Member] | Sovran HHF Storage Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	4
Florida [Member] | Sovran HHF Storage Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	3
Georgia [Member] | Sovran HHF Storage Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	1
Georgia [Member] | Life Storage-SERS Storage LLC [Member] | Additional Storage Facilities [Member]
Schedule of Equity Method Investments [Line Items]
Number of storage facilities acquired | Property	3		3
Payment for property acquisition			$ 39,100
Mortgages payable			$ 22,000
Kentucky [Member] | Sovran HHF Storage Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	2
Nevada [Member] | Sovran HHF Storage Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	5
New Jersey [Member] | Sovran HHF Storage Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	2
New Jersey [Member] | Sovran HHF Storage Holdings II LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	17
Ohio [Member] | Sovran HHF Storage Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	6
Pennsylvania [Member] | Sovran HHF Storage Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	1
Pennsylvania [Member] | Sovran HHF Storage Holdings II LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	3
Tennessee [Member] | Sovran HHF Storage Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	2
Texas [Member] | Sovran HHF Storage Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	20
Texas [Member] | Sovran HHF Storage Holdings II LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	10
Arizona, Nevada and Tennessee [Member] | Sovran HHF Storage Holdings LLC [Member] | Storage Facilities [Member]
Schedule of Equity Method Investments [Line Items]
Number of storage facilities acquired | Property			18
Payment for property acquisition			$ 330,000
Mortgages payable			$ 135,000
Number of properties collateral for debt | Property			16
California [Member] | 191 III Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of self-storage facilities | Property	6
California [Member] | 191 III Holdings LLC [Member] | Six Storage Facilities [Member]
Schedule of Equity Method Investments [Line Items]
Number of storage facilities acquired | Property			6
Payment for property acquisition			$ 104,100
Mortgages payable			$ 57,200