FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Assets
Derivative assets	$ 86.8	$ 17.9
Liabilities
Derivative liabilities	9.4	22.1
Fair value measurements on a recurring basis
Assets
Derivative assets	86.8	17.9
Investments held in rabbi trust	77.6	79.6
Liabilities
Derivative liabilities	9.4	22.1
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative assets	70.2	11.7
Investments held in rabbi trust	77.6	79.6
Liabilities
Derivative liabilities	0.0	7.7
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative assets	11.2	3.8
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	9.4	14.4
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative assets	5.4	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	74.4	13.7
Liabilities
Derivative liabilities	5.8	14.1
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative assets	70.2	11.7
Liabilities
Derivative liabilities	0.0	7.7
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative assets	4.2	2.0
Liabilities
Derivative liabilities	5.8	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	5.4	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	5.4	2.4
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative assets	7.0	1.8
Liabilities
Derivative liabilities	0.2	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	0.0
Fair value measurements on a recurring basis \| Coal contracts \| Level 2
Assets
Derivative assets	7.0	1.8
Liabilities
Derivative liabilities	0.2	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	0.0
Fair value measurements on a recurring basis \| Interest rate swaps
Liabilities
Derivative liabilities	3.4	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	3.4	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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