GOODWILL AND INTANGIBLES - INTANGIBLE LIABILITIES (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Finite-Lived Intangible Liabilities
Amortization	$ 1.9	$ 3.7
Period of amortization		5 years
Operating revenues
Amortization to be recorded in the next five years
2022	8.9	$ 8.9
2023	8.9	8.9
2024	8.9	8.9
2025	8.9	8.9
2026	8.9	8.9
Other operation and maintenance
Amortization to be recorded in the next five years
2022	0.2	0.2
2023	0.2	0.2
2024	0.2	0.2
2025	0.2	0.2
2026	0.2	0.2
WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	105.4	105.4	$ 88.4
Accumulated amortization	(5.3)	(5.3)	(1.6)
Net carrying amount	100.1	100.1	86.8
PPAs \| WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	93.1	93.1	76.1
Accumulated amortization	(3.2)	(3.2)	0.0
Net carrying amount	89.9	$ 89.9	76.1
PPAs \| Blooming Grove Wind Energy Center LLC, Tatanka Ridge Wind, LLC and Jayhawk
Finite-Lived Intangible Liabilities
Weighted average life		11 years
Proxy revenue swap \| WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	7.2	$ 7.2	7.2
Accumulated amortization	(1.7)	(1.7)	(1.3)
Net carrying amount	5.5	$ 5.5	5.9
Proxy revenue swap \| Upstream
Finite-Lived Intangible Liabilities
Weighted average life		8 years
Length of proxy revenue contract, in years		10 years
Interconnection agreements \| WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	5.1	$ 5.1	5.1
Accumulated amortization	(0.4)	(0.4)	(0.3)
Net carrying amount	$ 4.7	$ 4.7	$ 4.8
Interconnection agreements \| Tatanka Ridge Wind LLC and Bishop Hill Energy III LLC
Finite-Lived Intangible Liabilities
Weighted average life		19 years

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