FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Assets
Derivative assets	$ 40.1	$ 6.9
Liabilities
Derivative liabilities	15.5	28.9
Fair value measurements on a recurring basis
Assets
Derivative assets	40.1	6.9
Investments held in rabbi trust	72.6	85.3
Liabilities
Derivative liabilities	15.5	28.9
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative assets	32.4	1.4
Investments held in rabbi trust	72.6	85.3
Liabilities
Derivative liabilities	0.9	21.4
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative assets	3.7	2.4
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	14.6	7.5
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative assets	4.0	3.1
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	35.6	3.4
Liabilities
Derivative liabilities	4.8	22.7
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative assets	32.4	1.4
Liabilities
Derivative liabilities	0.9	21.4
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative assets	3.2	2.0
Liabilities
Derivative liabilities	3.9	1.3
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	4.0	3.1
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	4.0	3.1
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative assets	0.5	0.4
Liabilities
Derivative liabilities	2.4	0.2
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	0.5	0.4
Liabilities
Derivative liabilities	2.4	0.2
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	8.3	6.0
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	8.3	6.0
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 3
Liabilities
Derivative liabilities	$ 0.0	$ 0.0

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