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NONCONTROLLING INTERESTS - Interests in Consolidated Real Estate Joint Ventures (Details) $ in Thousands		1 Months Ended	3 Months Ended	6 Months Ended
	Apr. 16, 2021 USD ($)	Mar. 31, 2021 facility	Jun. 30, 2021 USD ($)	Jun. 30, 2021 USD ($) facility	Dec. 31, 2020 USD ($)
Interests in Consolidated Real Estate Joint Ventures
Assets			$ 4,771,232	$ 4,771,232	$ 4,778,142
Total assets			4,771,232	4,771,232	4,778,142
Liabilities			2,573,782	2,573,782	2,687,880
Total liabilities			2,573,782	2,573,782	2,687,880
Noncontrolling interests in subsidiaries			13,509	$ 13,509	$ 8,632
Reduction to additional paid in capital			5,104
VIE
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility				6
Assets			132,714	$ 132,714
Total assets			132,714	132,714
Liabilities			51,539	51,539
Total liabilities			51,539	$ 51,539
CS Vienna 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)				72.00%
Assets			19,802	$ 19,802
Total assets			19,802	19,802
Liabilities			8,615	8,615
Total liabilities			8,615	8,615
Carrying amount of mortgage loans			6,700	6,700
Amount of mortgage loan commitment			17,000	$ 17,000
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			20,283	$ 20,283
Total assets			20,283	20,283
Liabilities			10,604	10,604
Total liabilities			10,604	10,604
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				$ 9,800
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			19,190	$ 19,190
Total assets			19,190	19,190
Liabilities			12,674	12,674
Total liabilities			12,674	12,674
Carrying amount of mortgage loans			11,800	11,800
Amount of mortgage loan commitment			12,100	$ 12,100
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			21,891	$ 21,891
Total assets			21,891	21,891
Liabilities			13,866	13,866
Total liabilities			13,866	13,866
Carrying amount of mortgage loans			11,300	11,300
Amount of mortgage loan commitment			12,400	$ 12,400
SH3, LLC \| VIE
Interests in Consolidated Real Estate Joint Ventures
Number of self-storage facilities owned and operated \| facility		2		1
Ownership interest held by the entity (as a percent)				90.00%
Assets			39,334	$ 39,334
Total assets			39,334	39,334
Liabilities			191	191
Total liabilities			191	$ 191
C S Lock Up Anoka 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)	50.00%			50.00%
Contribution To Real Estate Venture	$ 3,400
Assets			12,214	$ 12,214
Total assets			12,214	12,214
Liabilities			5,589	5,589
Total liabilities			5,589	5,589
Carrying amount of mortgage loans			5,600	5,600
Amount of mortgage loan commitment			$ 6,100	$ 6,100
C S Lock Up Anoka LLC \| VIE \| Minnesota
Interests in Consolidated Real Estate Joint Ventures
Purchase Price	$ 12,000