FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Assets
Derivative asset	$ 3.8	$ 5.1
Liabilities
Derivative liability	0.1	1.3
Fair value measurements on a recurring basis
Assets
Derivative asset	3.8	5.1
Liabilities
Derivative liability		1.3
Fair value measurements on a recurring basis | Level 1
Assets
Derivative asset	1.3	0.7
Liabilities
Derivative liability		1.2
Fair value measurements on a recurring basis | Level 2
Assets
Derivative asset	0.5	0.0
Liabilities
Derivative liability		0.1
Fair value measurements on a recurring basis | Level 3
Assets
Derivative asset	2.0	4.4
Liabilities
Derivative liability		0.0
Fair value measurements on a recurring basis | Natural gas contracts
Assets
Derivative asset	1.3	0.7
Liabilities
Derivative liability		1.2
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative asset	1.3	0.7
Liabilities
Derivative liability		1.2
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability		0.0
Fair value measurements on a recurring basis | Natural gas contracts | Level 3
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability		0.0
Fair value measurements on a recurring basis | FTRs
Assets
Derivative asset	2.0	4.4
Fair value measurements on a recurring basis | FTRs | Level 1
Assets
Derivative asset	0.0	0.0
Fair value measurements on a recurring basis | FTRs | Level 2
Assets
Derivative asset	0.0	0.0
Fair value measurements on a recurring basis | FTRs | Level 3
Assets
Derivative asset	2.0	4.4
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative asset	0.5
Liabilities
Derivative liability	0.1	0.1
Fair value measurements on a recurring basis | Coal contracts | Level 1
Assets
Derivative asset	0.0
Liabilities
Derivative liability	0.0	0.0
Fair value measurements on a recurring basis | Coal contracts | Level 2
Assets
Derivative asset	0.5
Liabilities
Derivative liability	0.1	0.1
Fair value measurements on a recurring basis | Coal contracts | Level 3
Assets
Derivative asset	0.0
Liabilities
Derivative liability	$ 0.0	$ 0.0