Investment in Storage Facilities - Schedule of Acquired Facilities and Purchase Price of Facilities (Detail) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) Facility Property	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) Facility	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Investment Holdings [Line Items]
Number of Properties | Facility			15		33
Cash Paid			$ 208,700
Closing Costs Expensed	$ 788	$ 1,938	$ 1,369	$ 4,716	$ 7,359	$ 3,129	$ 4,328
Acquisitions 2015 [Member]
Investment Holdings [Line Items]
Number of Properties | Property			15
Purchase Price	209,006		$ 209,006
Cash Paid			208,703
Consideration paid Value of Operating Partnership Units Issued			2,148
Net Other Liabilities (Assets) Assumed	(1,845)		(1,845)
Land	56,692		56,692
Acquisition Date Building, Equipment, and Improvements	151,145		151,145
Fair Value In-Place Customers Leases	1,169		1,169
Closing Costs Expensed			$ 1,369
Connecticut [Member] | Date of Acquisition, 2/2/2015 [Member]
Investment Holdings [Line Items]
Number of Properties | Property			2
Date of Acquisition			Feb. 02, 2015
Purchase Price	61,116		$ 61,116
Cash Paid			62,377
Net Other Liabilities (Assets) Assumed	(1,261)		(1,261)
Land	19,389		19,389
Acquisition Date Building, Equipment, and Improvements	41,727		$ 41,727
New York [Member] | Date of Acquisition, 2/2/2015 [Member]
Investment Holdings [Line Items]
Number of Properties | Property			2
Date of Acquisition			Feb. 02, 2015
Purchase Price	57,900		$ 57,900
Cash Paid			59,103
Net Other Liabilities (Assets) Assumed	(1,203)		(1,203)
Land	10,084		10,084
Acquisition Date Building, Equipment, and Improvements	47,816		$ 47,816
Illinois [Member] | Date of Acquisition, 2/5/2015 [Member]
Investment Holdings [Line Items]
Number of Properties | Property			1
Date of Acquisition			Feb. 05, 2015
Purchase Price	6,800		$ 6,800
Cash Paid			6,652
Net Other Liabilities (Assets) Assumed	148		148
Land	2,579		2,579
Acquisition Date Building, Equipment, and Improvements	4,066		4,066
Fair Value In-Place Customers Leases	155		155
Closing Costs Expensed			$ 157
Illinois [Member] | Date of Acquisition, 3/9/2015 [Member]
Investment Holdings [Line Items]
Number of Properties | Property			1
Date of Acquisition			Mar. 09, 2015
Purchase Price	8,690		$ 8,690
Cash Paid			6,466
Consideration paid Value of Operating Partnership Units Issued			2,148
Net Other Liabilities (Assets) Assumed	76		76
Land	1,719		1,719
Acquisition Date Building, Equipment, and Improvements	6,971		$ 6,971
Florida [Member] | Date of Acquisition, 4/1/2015 [Member]
Investment Holdings [Line Items]
Number of Properties | Property			1
Date of Acquisition			Apr. 01, 2015
Purchase Price	6,290		$ 6,290
Cash Paid			6,236
Net Other Liabilities (Assets) Assumed	54		54
Land	1,793		1,793
Acquisition Date Building, Equipment, and Improvements	4,382		4,382
Fair Value In-Place Customers Leases	115		115
Closing Costs Expensed			$ 370
Florida [Member] | Date of Acquisition, 4/21/2015 [Member]
Investment Holdings [Line Items]
Number of Properties | Property			1
Date of Acquisition			Apr. 21, 2015
Purchase Price	8,750		$ 8,750
Cash Paid			8,687
Net Other Liabilities (Assets) Assumed	63		63
Land	2,118		2,118
Acquisition Date Building, Equipment, and Improvements	6,501		6,501
Fair Value In-Place Customers Leases	131		131
Closing Costs Expensed			$ 133
Florida [Member] | Date of Acquisition, 5/1/2015 [Member]
Investment Holdings [Line Items]
Number of Properties | Property			4
Date of Acquisition			May 01, 2015
Purchase Price	32,465		$ 32,465
Cash Paid			32,279
Net Other Liabilities (Assets) Assumed	186		186
Land	12,184		12,184
Acquisition Date Building, Equipment, and Improvements	19,672		19,672
Fair Value In-Place Customers Leases	609		609
Closing Costs Expensed			$ 558
Texas [Member] | Date of Acquisition, 4/16/2015 [Member]
Investment Holdings [Line Items]
Number of Properties | Property			1
Date of Acquisition			Apr. 16, 2015
Purchase Price	8,800		$ 8,800
Cash Paid			8,713
Net Other Liabilities (Assets) Assumed	87		87
Land	3,864		3,864
Acquisition Date Building, Equipment, and Improvements	4,777		4,777
Fair Value In-Place Customers Leases	159		159
Closing Costs Expensed			$ 151
Arizona [Member] | Date of Acquisition, 6/16/2015 [Member]
Investment Holdings [Line Items]
Number of Properties | Property			1
Date of Acquisition			Jun. 16, 2015
Purchase Price	7,904		$ 7,904
Cash Paid			7,904
Land	852		852
Acquisition Date Building, Equipment, and Improvements	7,052		$ 7,052
Massachusetts [Member] | Date of Acquisition, 6/19/2015 [Member]
Investment Holdings [Line Items]
Number of Properties | Property			1
Date of Acquisition			Jun. 19, 2015
Purchase Price	10,291		$ 10,291
Cash Paid			10,286
Net Other Liabilities (Assets) Assumed	5		5
Land	2,110		2,110
Acquisition Date Building, Equipment, and Improvements	$ 8,181		$ 8,181