FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Assets
Derivative assets	$ 33.2	$ 6.3
Liabilities
Derivative liabilities	0.4	4.1
Fair value measurements on a recurring basis
Assets
Derivative assets	33.2	6.3
Liabilities
Derivative liabilities		4.1
Fair value measurements on a recurring basis | Level 1
Assets
Derivative assets	23.2	3.0
Liabilities
Derivative liabilities		2.9
Fair value measurements on a recurring basis | Level 2
Assets
Derivative assets	7.4	2.2
Liabilities
Derivative liabilities		1.2
Fair value measurements on a recurring basis | Level 3
Assets
Derivative assets	2.6	1.1
Liabilities
Derivative liabilities		0.0
Fair value measurements on a recurring basis | Natural gas contracts
Assets
Derivative assets	24.4	3.8
Liabilities
Derivative liabilities	0.4	3.5
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative assets	23.2	3.0
Liabilities
Derivative liabilities	0.0	2.9
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative assets	1.2	0.8
Liabilities
Derivative liabilities	0.4	0.6
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	2.6	1.1
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	2.6	1.1
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative assets	6.2	1.4
Liabilities
Derivative liabilities		0.6
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	6.2	1.4
Liabilities
Derivative liabilities		0.6
Fair value measurements on a recurring basis | Coal contracts | Level 3
Assets
Derivative assets	$ 0.0	0.0
Liabilities
Derivative liabilities		$ 0.0