FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets
Derivative assets	$ 2.1	$ 4.2
Liabilities
Derivative liabilities	3.3	1.1
Fair value measurements on a recurring basis
Assets
Derivative assets	2.1	4.2
Liabilities
Derivative liabilities	3.3
Fair value measurements on a recurring basis | Level 1
Assets
Derivative assets	0.2	0.8
Liabilities
Derivative liabilities	3.0
Fair value measurements on a recurring basis | Level 2
Assets
Derivative assets	0.6	0.4
Liabilities
Derivative liabilities	0.3
Fair value measurements on a recurring basis | Level 3
Assets
Derivative assets	1.3	3.0
Liabilities
Derivative liabilities	0.0
Fair value measurements on a recurring basis | Natural gas contracts
Assets
Derivative assets	0.4	0.8
Liabilities
Derivative liabilities	3.2	1.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative assets	0.2	0.8
Liabilities
Derivative liabilities	3.0	1.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative assets	0.2	0.0
Liabilities
Derivative liabilities	0.2	0.0
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	1.3	3.0
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	1.3	3.0
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative assets	0.4	0.4
Liabilities
Derivative liabilities	0.1
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	0.4	0.4
Liabilities
Derivative liabilities	0.1
Fair value measurements on a recurring basis | Coal contracts | Level 3
Assets
Derivative assets	0.0	$ 0.0
Liabilities
Derivative liabilities	$ 0.0