GOODWILL AND INTANGIBLES - INTANGIBLE LIABILITIES (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets
Period of amortization	5 years
Operating revenues
Amortization to be recorded in the next five years
2022	$ 8.1
2023	8.1
2024	8.1
2025	8.1
2026	8.1
Other operation and maintenance
Amortization to be recorded in the next five years
2022	0.2
2023	0.2
2024	0.2
2025	0.2
2026	0.2
WECI
Finite-Lived Intangible Assets
Gross carrying amount	96.9	$ 88.4
Accumulated amortization	(3.5)	(1.6)
Net carrying amount	93.4	86.8
PPAs \| WECI
Finite-Lived Intangible Assets
Gross carrying amount	84.6	76.1
Accumulated amortization	(1.6)	0.0
Net carrying amount	$ 83.0	76.1
PPAs \| Blooming Grove Wind Energy Center LLC, Tatanka Ridge Wind, LLC and Jayhawk
Finite-Lived Intangible Assets
Weighted average useful life	11 years
Proxy revenue swap \| WECI
Finite-Lived Intangible Assets
Gross carrying amount	$ 7.2	7.2
Accumulated amortization	(1.5)	(1.3)
Net carrying amount	$ 5.7	5.9
Proxy revenue swap \| Upstream
Finite-Lived Intangible Assets
Weighted average useful life	8 years
Length of proxy revenue contract, in years	10 years
Interconnection agreements \| WECI
Finite-Lived Intangible Assets
Gross carrying amount	$ 5.1	5.1
Accumulated amortization	(0.4)	(0.3)
Net carrying amount	$ 4.7	$ 4.8
Interconnection agreements \| Tatanka Ridge Wind LLC and Bishop Hill Energy III LLC
Finite-Lived Intangible Assets
Weighted average useful life	20 years

Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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 of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount before 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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Period of amortization over a future period of time

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