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NONCONTROLLING INTERESTS - Interests in Consolidated Real Estate Joint Ventures (Details) $ in Thousands			3 Months Ended		12 Months Ended
	May 30, 2019 USD ($)	Mar. 07, 2019 USD ($)	Mar. 31, 2020 USD ($) facility	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	May 29, 2019 USD ($)
Interests in Consolidated Real Estate Joint Ventures
Assets			$ 4,002,762		$ 4,029,545
Liabilities			2,158,627		2,160,121
Purchase Price					118,300
Elimination of noncontrolling interest in subsidiaries due to acquisition				$ 10,000
Noncontrolling interests in subsidiaries			$ 7,969		$ 7,990
Reduction to additional paid in capital				$ 4,828
VIE
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility			6
Assets			$ 100,526
Liabilities			$ 23,397
CS Valley Forge Village Storage LLC \| VIE
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)			70.00%
Assets			$ 7,022
Liabilities			$ 1,592
Shirlington Rd II LLC \| VIE
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%
Assets			$ 10,429
Liabilities			998
CS 2087 Hempstead Tpk LLC
Interests in Consolidated Real Estate Joint Ventures
Option to call ownership interest of another member			6,600
Accretion liability			$ 3,600
CS 2087 Hempstead Tpk LLC \| VIE
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%
Assets			$ 13,543
Liabilities			$ 3,941
CS SDP Newtonville LLC \| VIE
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%
Assets			$ 12,461
Liabilities			5,938
CS 1158 McDonald Ave LLC
Interests in Consolidated Real Estate Joint Ventures
Option to call ownership interest of another member			10,000
Accretion liability			$ 9,900
CS 1158 McDonald Ave LLC \| VIE
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%
Assets			$ 42,312
Liabilities			$ 10,687
Shirlington Rd LLC
Interests in Consolidated Real Estate Joint Ventures
Elimination of noncontrolling interest in subsidiaries due to acquisition		$ 10,000
Noncontrolling interests in subsidiaries		0
Reduction to additional paid in capital		9,700
Amount of debt repaid		12,200
Shirlington Rd LLC \| VIE
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%
Assets			$ 14,759
Liabilities			$ 241
CS SJM E 92nd Street LLC
Interests in Consolidated Real Estate Joint Ventures
Percentage of the Variable Interest Entity sold	90.00%
Carrying amount of mortgage loans							$ 1,700
Proceeds from sale of real estate, net	$ 3,700
Note receivable						$ 1,000
Amount of debt repaid	700
Increase (Decrease) Operating Lease Liability	(14,600)
Increase (decrease) in operating lease, right-of-use asset	$ (13,400)
VIE
Interests in Consolidated Real Estate Joint Ventures
Period of option to call ownership interest of another member			1 year
Period of option to put ownership interest in venture			1 year
Payment made for acquisition of interest in real estate ventures		$ 4,800
VIE \| CS Valley Forge Village Storage LLC
Interests in Consolidated Real Estate Joint Ventures
Amount of mortgage loan commitment			$ 12,400
VIE \| Shirlington Rd II LLC
Interests in Consolidated Real Estate Joint Ventures
Percentage of interest acquired		10.00%
VIE \| CS 2087 Hempstead Tpk LLC
Interests in Consolidated Real Estate Joint Ventures
Option to put ownership interest in the venture			6,600
VIE \| CS SDP Newtonville LLC
Interests in Consolidated Real Estate Joint Ventures
Carrying amount of mortgage loans			5,000
Amount of mortgage loan commitment			12,100
VIE \| CS 1158 McDonald Ave LLC
Interests in Consolidated Real Estate Joint Ventures
Option to put ownership interest in the venture			$ 10,000
VIE \| Shirlington Rd LLC
Interests in Consolidated Real Estate Joint Ventures
Percentage of interest acquired		10.00%