REAL ESTATE AND ACQUIRED IN PLACE LEASE VALUE - Schedule of Consolidated Acquisitions (Details) $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($) property	Dec. 31, 2019 USD ($) property
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Land	$ 13.4	$ 76.1
Building	95.1	193.9
Acquired in place lease values	0.5	43.5
Investment debt	38.7	214.8
KWH Shareholders' Equity	147.8	98.7
Western U.S. \| Multifamily
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Land	13.4
Building	53.6
Acquired in place lease values	0.5
Investment debt	38.7
KWH Shareholders' Equity	$ 106.3
Western U.S. \| Multifamily \| Multifamily
Business Acquisition [Line Items]
Number of properties acquired \| property	1
Western U.S. \| Multifamily and Commercial
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Land		39.9
Building		123.0
Acquired in place lease values		37.2
Investment debt		112.2
KWH Shareholders' Equity		$ 87.9
Western U.S. \| Multifamily and Commercial \| Multifamily
Business Acquisition [Line Items]
Number of properties acquired \| property		1
Western U.S. \| Multifamily and Commercial \| Commercial
Business Acquisition [Line Items]
Number of properties acquired \| property		1
United Kingdom \| Industrial
Business Acquisition [Line Items]
Number of properties acquired \| property	1
United Kingdom \| Commercial and Industrial Properties
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Land	$ 0.0	$ 29.0
Building	40.2	39.2
Acquired in place lease values	0.0	2.9
Investment debt	0.0	58.6
KWH Shareholders' Equity	40.2	$ 12.5
United Kingdom \| Commercial and Industrial Properties \| Commercial
Business Acquisition [Line Items]
Number of properties acquired \| property		1
United Kingdom \| Commercial and Industrial Properties \| Industrial
Business Acquisition [Line Items]
Number of properties acquired \| property		1
Ireland \| Commercial Property Acquired
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Land	0.0	$ 7.2
Building	1.3	31.7
Acquired in place lease values	0.0	3.4
Investment debt	0.0	44.0
KWH Shareholders' Equity	$ 1.3	$ (1.7)
Ireland \| Commercial Property Acquired \| Commercial
Business Acquisition [Line Items]
Number of properties acquired \| property	1	1

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