Investment in Joint Ventures - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015 USD ($) Property	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) Property	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) Property	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($) Property	Dec. 31, 2011 USD ($) Property	Dec. 31, 2008 USD ($) Property
Schedule of Equity Method Investments [Line Items]
Equity in income of joint ventures	$ 853,000	$ 433,000	$ 1,499,000	$ 892,000	$ 2,086,000	$ 1,948,000	$ 936,000
Sovran HHF Storage Holdings II LLC [Member]
Schedule of Equity Method Investments [Line Items]
Investment at carrying value	$ 12,500,000		$ 12,500,000		12,600,000
Payment for property acquisition							29,000,000	$ 166,100,000
Contribution of company to joint venture as share in capital							$ 2,400,000	$ 12,800,000
Percentage of variable interest ownership in unconsolidated joint venture			15.00%
Number of properties owned and managed under fifteen-percent stake joint venture | Property	30		30				10	20
Sovran HHF Storage Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Percentage ownership in unconsolidated joint venture	20.00%		20.00%
Investment at carrying value	$ 45,100,000		$ 45,100,000		45,200,000
Payment for property acquisition					$ 187,200,000				$ 171,500,000
Number of properties owned and managed under twenty-percent stake joint venture | Property	39		39						25
Additional number of properties owned and managed under twenty-percent stake joint venture | Property					14
Contribution of company to joint venture as share in capital					$ 28,600,000		$ 1,200,000		$ 18,600,000
Receipt of return of capital distribution						$ 3,400,000
Excess of investment over net asset due to capitalization of acquisition related costs	$ 1,700,000		$ 1,700,000
Other-than-temporary impairment recorded on investment			0
Sovran HHF and Sovran HHF II [Member]
Schedule of Equity Method Investments [Line Items]
Management and call center fee as a percentage of revenues	7.00%
Management and call center fee earned	$ 1,200,000	900,000	2,400,000	1,800,000
Equity in income of joint ventures	$ 807,000	367,000	$ 1,395,000	793,000
Iskalo Office Holdings, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Percentage ownership in unconsolidated joint venture	49.00%		49.00%
Equity in income of joint ventures	$ 46,000	32,000	$ 104,000	64,000
Investment liability at carrying value	(600,000)		(600,000)		$ (500,000)
Rent paid during the period	$ 259,000	$ 252,000	$ 534,000	$ 503,000