Investment in Storage Facilities - Summary of Revenues and Expenses Related to Sale of Properties Included in Company's Consolidated Statements of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total revenues	$ 90,726	$ 80,444	$ 176,134	$ 155,902	$ 326,080	$ 273,507	$ 234,082
Property operations and maintenance expense	(19,486)	(17,993)	(40,046)	(36,478)	(75,333)	(66,119)	(59,011)
Real estate tax expense	(9,145)	(8,055)	(18,064)	(16,121)	(32,097)	(26,496)	(22,076)
Depreciation and amortization expense	$ (14,584)	$ (12,481)	(28,766)	$ (24,423)	(51,749)	$ (45,233)	$ (40,542)
Loss on sale of storage facilities			(7)		$ 5,176
Properties Sold In Two Thousand And Fifteen Member
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total revenues			40
Property operations and maintenance expense			(16)
Real estate tax expense			(5)
Depreciation and amortization expense			(9)
Loss on sale of storage facilities			(7)
Total income from discontinued operations			$ 3