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NONCONTROLLING INTERESTS - Interests in Consolidated Real Estate Joint Ventures (Details) $ in Thousands, £ in Millions			1 Months Ended	12 Months Ended
	Oct. 02, 2015 GBP (£)	Oct. 02, 2015 USD ($)	Jun. 30, 2014 USD ($) facility	Dec. 31, 2017 USD ($) facility	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2008 property	Dec. 15, 2017	Dec. 31, 2014 GBP (£)	Dec. 31, 2014 USD ($)
Interests in Consolidated Real Estate Joint Ventures
Interest rate (as a percent)								4.375%
Gain (loss) from sale of land						$ 17,567
Proceeds from sales of facilities, net						9,041
Gain from sale of real estate, net						17,567
Elimination of noncontrolling interest in subsidiaries due to acquisition				$ 9,033
Reduction to additional paid in capital				1,058	$ 4,799	$ 178
CS 1158 McDonald Ave LLC
Interests in Consolidated Real Estate Joint Ventures
Option to call ownership interest of another member				10,000
Accretion liability				2,200
CS 160 East 22nd Street LLC
Interests in Consolidated Real Estate Joint Ventures
Option to call ownership interest of another member				11,500
Accretion liability				3,300
2225 46th Street LLC
Interests in Consolidated Real Estate Joint Ventures
Option to call ownership interest of another member				14,200
Accretion liability				8,200
2880 Exterior St. LLC
Interests in Consolidated Real Estate Joint Ventures
Option to call ownership interest of another member				37,800
Accretion liability				28,900
3068 Cropsey Ave LLC
Interests in Consolidated Real Estate Joint Ventures
Option to call ownership interest of another member				20,400
Accretion liability				20,400
186 Jamaica Ave LLC
Interests in Consolidated Real Estate Joint Ventures
Carrying amount of mortgage loans				$ 12,800
VIE
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility				10
Total assets				$ 286,211
Total liabilities				$ 130,892
Period of option to call ownership interest of another member				1 year
Period of option to put ownership interest in venture				1 year
VIE \| CS 1158 McDonald Ave LLC
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility				1
Ownership interest held by the entity (as a percent)				51.00%
Total assets				$ 18,472
Total liabilities				2,429
Option to put ownership interest in the venture				$ 10,000
VIE \| CS SJM E 92nd Street LLC
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility				1
Ownership interest held by the entity (as a percent)				90.00%
Total assets				$ 1,366
Total liabilities				1,096
Amount of mortgage loan commitment				$ 6,200
VIE \| CS 160 East 22nd Street LLC
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility				1
Ownership interest held by the entity (as a percent)				51.00%
Total assets				$ 5,533
Total liabilities				3,382
Option to put ownership interest in the venture				$ 11,500
VIE \| 2225 46th Street LLC
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility				1
Ownership interest held by the entity (as a percent)				51.00%
Total assets				$ 27,130
Total liabilities				9,551
Option to put ownership interest in the venture				$ 14,200
VIE \| CS SDP Waltham LLC
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility				1
Ownership interest held by the entity (as a percent)				90.00%
Total assets				$ 5,981
Total liabilities				704
Amount of mortgage loan commitment				$ 10,800
VIE \| 2880 Exterior St. LLC
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility				1
Ownership interest held by the entity (as a percent)				51.00%
Total assets				$ 62,763
Total liabilities				31,575
Option to put ownership interest in the venture				$ 37,800
VIE \| 3068 Cropsey Ave LLC
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility				1
Ownership interest held by the entity (as a percent)				51.00%
Total assets				$ 47,952
Total liabilities				22,189
Option to put ownership interest in the venture				$ 20,400
VIE \| 444 55th Street Holdings, LLC
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility				1
Ownership interest held by the entity (as a percent)				90.00%
Total assets				$ 82,216
Total liabilities				33,858
Assumed mortgage debt, at fair value				35,000
Outstanding principal balance of mortgage debt assumed on acquisitions				32,500
Premium on debt assumed on one of the acquisitions				$ 2,500
Interest rate (as a percent)				4.68%
VIE \| 186 Jamaica Ave LLC
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility				1
Ownership interest held by the entity (as a percent)				90.00%
Total assets				$ 18,478
Total liabilities				$ 13,289
VIE \| Shirlington Rd LLC
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility				1
Ownership interest held by the entity (as a percent)				90.00%
Total assets				$ 16,320
Total liabilities				12,819
USIFB, LLP
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| property							2
Ownership interest held by the entity (as a percent)	97.00%	97.00%				97.00%	97.00%
Carrying amount of mortgage loans									£ 4.1	$ 6,800
Gain (loss) from sale of land		$ 3,000
Number of assets sold \| facility			1
Proceeds from sales of facilities, net			$ 7,000
Gain from sale of real estate, net		3,000
Foreign currency translation loss		1,200
Aggregate sale price	£ 6.5	$ 9,900
SRLLC
Interests in Consolidated Real Estate Joint Ventures
Carrying amount of mortgage loans				12,700
Amount of mortgage loan commitment				$ 14,600