Investment in Storage Facilities - Additional Information (Detail)	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2015 USD ($) Facility	Mar. 31, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) Facility Property Store	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) Facility	Feb. 02, 2015 Facility
Investment Holdings [Line Items]
Number of facilities acquired | Facility				15		33
Stores acquired at certificate of occupancy | Store				3
Number of facilities acquired included in operations | Facility	11			11
Cash paid for properties acquired				$ 208,700,000
Deposits paid at the time of contract	$ 250,000			250,000
Issuance of Operating Partnership Units		$ 2,100,000		2,100,000
Net other liabilities assumed				600,000
Settlement of a straight-line rent liability				$ 2,400,000
Amortization period for in-place customer leases on a straight-line basis				12 months
Amortization expense related to in-place customer leases	1,000,000		$ 800,000	$ 2,000,000	$ 1,500,000
Amortization expense expected for 2015	3,000,000			3,000,000
Amortization expense expected for 2016	341,000			$ 341,000
Number of properties sold | Property				1
Carrying value of properties sold	$ 698,000			$ 698,000
Net proceeds from the sale of storage facility				711,000
Loss on sale of storage facility				(7,000)		$ 5,176,000
One Property [Member]
Investment Holdings [Line Items]
Net proceeds from the sale of storage facility				$ 691,000
Connecticut and New York [Member]
Investment Holdings [Line Items]
Number of facilities acquired and leased since November 1, 2013 | Facility							4