FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets
Derivative asset	$ 11.0	$ 4.6
Liabilities
Derivative liability	0.1	2.4
Fair value measurements on a recurring basis
Assets
Derivative asset	11.0	4.6
Liabilities
Derivative liability		2.4
Fair value measurements on a recurring basis | Level 1
Assets
Derivative asset	1.8	1.4
Liabilities
Derivative liability		2.0
Fair value measurements on a recurring basis | Level 2
Assets
Derivative asset	0.5	0.8
Liabilities
Derivative liability		0.4
Fair value measurements on a recurring basis | Level 3
Assets
Derivative asset	8.7	2.4
Liabilities
Derivative liability		0.0
Fair value measurements on a recurring basis | Natural gas contracts
Assets
Derivative asset	1.5	0.6
Liabilities
Derivative liability	0.1	2.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative asset	1.5	0.5
Liabilities
Derivative liability	0.1	2.0
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative asset	0.0	0.1
Liabilities
Derivative liability	0.0	0.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 3
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Fair value measurements on a recurring basis | Petroleum products contracts
Assets
Derivative asset	0.3	0.9
Fair value measurements on a recurring basis | Petroleum products contracts | Level 1
Assets
Derivative asset	0.3	0.9
Fair value measurements on a recurring basis | Petroleum products contracts | Level 2
Assets
Derivative asset	0.0	0.0
Fair value measurements on a recurring basis | Petroleum products contracts | Level 3
Assets
Derivative asset	0.0	0.0
Fair value measurements on a recurring basis | FTRs
Assets
Derivative asset	8.7	2.4
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	8.7	2.4
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative asset	0.5	0.7
Liabilities
Derivative liability		0.3
Fair value measurements on a recurring basis | Coal contracts | Level 1
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability		0.0
Fair value measurements on a recurring basis | Coal contracts | Level 2
Assets
Derivative asset	0.5	0.7
Liabilities
Derivative liability		0.3
Fair value measurements on a recurring basis | Coal contracts | Level 3
Assets
Derivative asset	$ 0.0	0.0
Liabilities
Derivative liability		$ 0.0