NONCONTROLLING INTERESTS - Interests in Consolidated Real Estate Joint Ventures (Details) $ in Thousands	3 Months Ended
Mar. 31, 2017 USD ($) facility
Interests in Consolidated Real Estate Joint Ventures
Reduction to additional paid in capital	$ 410
2225 46th Street LLC
Interests in Consolidated Real Estate Joint Ventures
Option to call ownership interest of another member	14,200
Accretion liability	3,400
2880 Exterior St. LLC
Interests in Consolidated Real Estate Joint Ventures
Option to call ownership interest of another member	37,800
Accretion liability	18,900
3068 Cropsey Ave LLC
Interests in Consolidated Real Estate Joint Ventures
Option to call ownership interest of another member	20,400
Accretion liability	$ 13,600
VIE
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility	9
Total assets	$ 223,699
Total liabilities	$ 111,023
Period of option to call ownership interest of another member	1 year
Period of option to put ownership interest in venture	1 year
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	$ 18,128
Total liabilities	3,425
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	$ 3,575
Total liabilities	$ 1
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	$ 587
Total liabilities	$ 568
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	$ 40,906
Total liabilities	19,572
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	$ 29,703
Total liabilities	14,994
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	$ 82,414
Total liabilities	37,077
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 \| CS SNL New York 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	$ 14,076
Total liabilities	$ 9,960
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	$ 18,017
Total liabilities	$ 12,535
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	$ 16,293
Total liabilities	12,891
SRLLC
Interests in Consolidated Real Estate Joint Ventures
Carrying amount of mortgage loans	12,700
Amount of mortgage loan commitment	14,600
SNL
Interests in Consolidated Real Estate Joint Ventures
Carrying amount of mortgage loans	9,800
SNL II
Interests in Consolidated Real Estate Joint Ventures
Carrying amount of mortgage loans	12,500
Amount of mortgage loan commitment	12,800
Watham
Interests in Consolidated Real Estate Joint Ventures
Amount of mortgage loan commitment	$ 10,800

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