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NONCONTROLLING INTERESTS - Interests in Consolidated Real Estate Joint Ventures (Details) $ in Thousands			3 Months Ended			9 Months Ended	18 Months Ended
	May 30, 2019 USD ($)	Mar. 07, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2020 USD ($) facility	Jun. 30, 2020	Dec. 31, 2019 USD ($)	May 29, 2019 USD ($)
Interests in Consolidated Real Estate Joint Ventures
Ownership interest held by the entity (as a percent)							100.00%
Assets						$ 4,028,570		$ 4,029,545
Liabilities						2,214,591		2,160,121
Elimination of noncontrolling interest in subsidiaries due to acquisition			$ 2,618	$ 27,618	$ 10,000
Noncontrolling interests in subsidiaries						$ 7,944		$ 7,990
Reduction to additional paid in capital			2,548		$ 4,828
VIE
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility						6
Assets						$ 86,613
Liabilities						$ 25,400
CS 750 W Merrick 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)						51.00%
Assets						$ 9,228
Liabilities						$ 2,501
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						$ 13,479
Liabilities						$ 5,155
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						$ 16,167
Liabilities						$ 1,533
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						$ 17,508
Liabilities						$ 6,745
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						$ 15,820
Liabilities						$ 9,300
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						$ 14,411
Liabilities						166
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
Payment made for acquisition of interest in real estate ventures		$ 4,800
VIE \| CS 750 W Merrick Rd LLC
Interests in Consolidated Real Estate Joint Ventures
Option to put ownership interest in the venture						$ 17,100
Period of option to call ownership interest of another member						1 year
Period of option to put ownership interest in venture						2 years
Option to call ownership interest of another member						$ 17,100
Initiation period for the call option						12 months
Accretion liability						$ 2,400
VIE \| CS Valley Forge Village Storage LLC
Interests in Consolidated Real Estate Joint Ventures
Carrying amount of mortgage loans						1,800
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
Option to call ownership interest of another member						6,600
Accretion liability						5,100
VIE \| CS SDP Newtonville LLC
Interests in Consolidated Real Estate Joint Ventures
Carrying amount of mortgage loans						7,500
Amount of mortgage loan commitment						$ 12,100
VIE \| Shirlington Rd LLC
Interests in Consolidated Real Estate Joint Ventures
Percentage of interest acquired		10.00%