GOODWILL AND INTANGIBLES - INTANGIBLE LIABILITIES (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2021	Dec. 31, 2020
Finite-Lived Intangible Liabilities
Amortization	$ 1.9	$ 5.6
Period of amortization		5 years
Amortization to be recorded in operating revenues
Amortization to be recorded in the next five years
2022	8.1	$ 8.1
2023	8.1	8.1
2024	8.1	8.1
2025	8.1	8.1
2026	8.1	8.1
Amortization to be recorded in 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	96.6	96.6	$ 88.4
Accumulated amortization	(7.2)	(7.2)	(1.6)
Net carrying amount	89.4	89.4	86.8
PPAs \| WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	84.3	84.3	76.1
Accumulated amortization	(4.9)	(4.9)	0.0
Net carrying amount	79.4	$ 79.4	76.1
PPAs \| Blooming Grove , Tatanka Ridge, 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	(1.9)	(1.9)	(1.3)
Net carrying amount	5.3	$ 5.3	5.9
Proxy revenue swap \| Upstream
Finite-Lived Intangible Liabilities
Weighted average life		7 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 and Bishop Hill III
Finite-Lived Intangible Liabilities
Weighted average life		19 years

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Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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