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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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GOODWILL AND INTANGIBLES - INTANGIBLE LIABILITIES (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Finite-Lived Intangible Liabilities
Net carrying amount	$ 581.4		$ 594.8
Amortization	$ 13.4	$ 10.4
Period of amortization	5 years
Amortization to be recorded as an increase to operating revenues
Amortization to be recorded in the next five years
2024	$ 53.4
2025	53.4
2026	53.4
2027	53.4
2028	53.4
Amortization to be recorded as a decrease to other operation and maintenance
Amortization to be recorded in the next five years
2024	0.2
2025	0.2
2026	0.2
2027	0.2
2028	0.2
WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	665.8		665.8
Accumulated amortization	(84.4)		(71.0)
Net carrying amount	581.4		594.8
PPAs \| WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	653.9		653.9
Accumulated amortization	(79.8)		(66.6)
Net carrying amount	$ 574.1		587.3
PPAs \| Blooming Grove , Tatanka Ridge, Jayhawk, Thunderhead, Samson I, and Sapphire Sky
Finite-Lived Intangible Liabilities
Weighted average remaining useful life	11 years
Proxy revenue swap \| WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	$ 7.2		7.2
Accumulated amortization	(3.6)		(3.5)
Net carrying amount	$ 3.6		3.7
Proxy revenue swap \| Upstream
Finite-Lived Intangible Liabilities
Weighted average remaining useful life	5 years
Length of proxy revenue contract, in years	10 years
Interconnection agreements \| WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	$ 4.7		4.7
Accumulated amortization	(1.0)		(0.9)
Net carrying amount	$ 3.7		$ 3.8
Interconnection agreements \| Tatanka Ridge and Bishop Hill III
Finite-Lived Intangible Liabilities
Weighted average remaining useful life	17 years