FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Assets
Derivative assets	$ 270.1	$ 17.9
Liabilities
Derivative liabilities	22.3	22.1
Fair value measurements on a recurring basis
Assets
Derivative assets	270.1	17.9
Investments held in rabbi trust	77.8	79.6
Liabilities
Derivative liabilities	22.3	22.1
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative assets	209.6	11.7
Investments held in rabbi trust	77.8	79.6
Liabilities
Derivative liabilities	0.1	7.7
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative assets	56.8	3.8
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	22.2	14.4
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative assets	3.7	2.4
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis \| Natural gas contracts
Assets
Derivative assets	222.2	13.7
Liabilities
Derivative liabilities	20.6	14.1
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative assets	209.6	11.7
Liabilities
Derivative liabilities	0.1	7.7
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative assets	12.6	2.0
Liabilities
Derivative liabilities	20.5	6.4
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 3
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis \| FTRs
Assets
Derivative assets	3.7	2.4
Fair value measurements on a recurring basis \| FTRs \| Level 1
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis \| FTRs \| Level 2
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis \| FTRs \| Level 3
Assets
Derivative assets	3.7	2.4
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative assets	44.2	1.8
Liabilities
Derivative liabilities		1.2
Fair value measurements on a recurring basis \| Coal contracts \| Level 1
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities		0.0
Fair value measurements on a recurring basis \| Coal contracts \| Level 2
Assets
Derivative assets	44.2	1.8
Liabilities
Derivative liabilities		1.2
Fair value measurements on a recurring basis \| Coal contracts \| Level 3
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities		0.0
Fair value measurements on a recurring basis \| Interest rate swaps
Liabilities
Derivative liabilities	1.7	6.8
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 1
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 2
Liabilities
Derivative liabilities	1.7	6.8
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 3
Liabilities
Derivative liabilities	$ 0.0	$ 0.0

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