FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Assets
Derivative assets	$ 15.0	$ 3.3
Liabilities
Derivative liabilities	0.3	2.4
Fair value measurements on a recurring basis
Assets
Derivative assets	15.0	3.3
Liabilities
Derivative liabilities	0.3	2.4
Fair value measurements on a recurring basis | Level 1
Assets
Derivative assets	11.1	1.6
Liabilities
Derivative liabilities	0.0	1.6
Fair value measurements on a recurring basis | Level 2
Assets
Derivative assets	1.1	0.5
Liabilities
Derivative liabilities	0.3	0.8
Fair value measurements on a recurring basis | Level 3
Assets
Derivative assets	2.8	1.2
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis | Natural gas contracts
Assets
Derivative assets	11.4	1.7
Liabilities
Derivative liabilities	0.2	1.9
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative assets	11.1	1.6
Liabilities
Derivative liabilities	0.0	1.6
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative assets	0.3	0.1
Liabilities
Derivative liabilities	0.2	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	2.8	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	2.8	1.2
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative assets	0.8	0.4
Liabilities
Derivative liabilities	0.1	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	0.0
Fair value measurements on a recurring basis | Coal contracts | Level 2
Assets
Derivative assets	0.8	0.4
Liabilities
Derivative liabilities	0.1	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	$ 0.0