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NONCONTROLLING INTERESTS - Interests in Consolidated Real Estate Joint Ventures (Details) $ in Thousands			3 Months Ended		6 Months Ended	18 Months Ended
	May 30, 2019 USD ($)	Mar. 07, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2020 USD ($) facility	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	May 29, 2019 USD ($)
Interests in Consolidated Real Estate Joint Ventures
Ownership interest held by the entity (as a percent)						51.00%
Assets					$ 4,024,340	$ 4,024,340	$ 4,029,545
Liabilities					2,204,216	2,204,216	2,160,121
Elimination of noncontrolling interest in subsidiaries due to acquisition			$ 27,618	$ 10,000
Noncontrolling interests in subsidiaries					$ 7,965	7,965	$ 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					$ 109,155	109,155
Liabilities					$ 26,695	26,695
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					$ 10,000	10,000
Liabilities					$ 1,916	1,916
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					$ 13,377	13,377
Liabilities					1,440	1,440
CS 2087 Hempstead Tpk LLC
Interests in Consolidated Real Estate Joint Ventures
Option to call ownership interest of another member					6,600
Accretion liability					$ 4,400
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					$ 14,714	14,714
Liabilities					$ 4,980	4,980
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					$ 13,709	13,709
Liabilities					7,188	7,188
CS 1158 McDonald Ave LLC
Interests in Consolidated Real Estate Joint Ventures
Option to call ownership interest of another member					10,000
Accretion liability					$ 10,000
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					$ 42,715	42,715
Liabilities					$ 11,004	11,004
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					$ 14,640	14,640
Liabilities					$ 167	167
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	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					6,000	6,000
Amount of mortgage loan commitment					12,100	$ 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%