GOODWILL AND INTANGIBLES - INTANGIBLE LIABILITIES (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Finite-Lived Intangible Liabilities
Net carrying amount	$ 554.6		$ 554.6		$ 594.8
Amortization	13.4	$ 13.4	$ 40.2	$ 37.2
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		$ 53.4
2025	53.4		53.4
2026	53.4		53.4
2027	53.4		53.4
2028	53.4		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		0.2
2025	0.2		0.2
2026	0.2		0.2
2027	0.2		0.2
2028	0.2		0.2
WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	665.8		665.8		665.8
Accumulated amortization	(111.2)		(111.2)		(71.0)
Net carrying amount	554.6		554.6		594.8
PPAs | WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	653.9		653.9		653.9
Accumulated amortization	(106.1)		(106.1)		(66.6)
Net carrying amount	547.8		$ 547.8		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		7.2
Accumulated amortization	(4.0)		(4.0)		(3.5)
Net carrying amount	$ 3.2		$ 3.2		3.7
Proxy revenue swap | Upstream
Finite-Lived Intangible Liabilities
Weighted average remaining useful life			4 years
Length of proxy revenue contract, in years	10 years		10 years
Interconnection agreements | WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	$ 4.7		$ 4.7		4.7
Accumulated amortization	(1.1)		(1.1)		(0.9)
Net carrying amount	$ 3.6		$ 3.6		$ 3.8
Interconnection agreements | Tatanka Ridge and Bishop Hill III
Finite-Lived Intangible Liabilities
Weighted average remaining useful life			16 years