GOODWILL AND INTANGIBLES - INTANGIBLE LIABILITIES (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Finite-Lived Intangible Liabilities
Amortization	$ 2.2	$ 1.9	$ 6.5	$ 5.6
Period of amortization			5 years
Amortization to be recorded as an increase to operating revenues
Amortization to be recorded in the next five years
2022	10.6		$ 10.6
2023	20.9		20.9
2024	20.9		20.9
2025	20.9		20.9
2026	20.9		20.9
Amortization to be recorded as a decrease to 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	224.8		224.8		$ 99.8
Accumulated amortization	(15.6)		(15.6)		(9.1)
Net carrying amount	209.2		209.2		90.7
PPAs | WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	212.9		212.9		87.9
Accumulated amortization	(12.3)		(12.3)		(6.5)
Net carrying amount	200.6		$ 200.6		81.4
PPAs | Blooming Grove , Tatanka Ridge, Jayhawk, and Thunderhead
Finite-Lived Intangible Liabilities
Weighted average life			10 years
Proxy revenue swap | WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	7.2		$ 7.2		7.2
Accumulated amortization	(2.6)		(2.6)		(2.1)
Net carrying amount	4.6		$ 4.6		5.1
Proxy revenue swap | Upstream
Finite-Lived Intangible Liabilities
Weighted average life			6 years
Length of proxy revenue contract, in years			10 years
Interconnection agreements | WECI
Finite-Lived Intangible Liabilities
Gross carrying amount	4.7		$ 4.7		4.7
Accumulated amortization	(0.7)		(0.7)		(0.5)
Net carrying amount	$ 4.0		$ 4.0		$ 4.2
Interconnection agreements | Tatanka Ridge and Bishop Hill III
Finite-Lived Intangible Liabilities
Weighted average life			18 years