FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Assets
Derivative assets	$ 57.2	$ 3.3
Liabilities
Derivative liabilities	0.4	2.4
Fair value measurements on a recurring basis
Assets
Derivative assets	57.2	3.3
Liabilities
Derivative liabilities		2.4
Fair value measurements on a recurring basis | Level 1
Assets
Derivative assets	37.6	1.6
Liabilities
Derivative liabilities		1.6
Fair value measurements on a recurring basis | Level 2
Assets
Derivative assets	17.5	0.5
Liabilities
Derivative liabilities		0.8
Fair value measurements on a recurring basis | Level 3
Assets
Derivative assets	2.1	1.2
Liabilities
Derivative liabilities		0.0
Fair value measurements on a recurring basis | Natural gas contracts
Assets
Derivative assets	39.7	1.7
Liabilities
Derivative liabilities	0.4	1.9
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative assets	37.6	1.6
Liabilities
Derivative liabilities	0.0	1.6
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative assets	2.1	0.1
Liabilities
Derivative liabilities	0.4	0.3
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	2.1	1.2
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	2.1	1.2
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative assets	15.4	0.4
Liabilities
Derivative liabilities		0.5
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	15.4	0.4
Liabilities
Derivative liabilities		0.5
Fair value measurements on a recurring basis | Coal contracts | Level 3
Assets
Derivative assets	$ 0.0	0.0
Liabilities
Derivative liabilities		$ 0.0