FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Assets
Derivative asset	$ 3.7	$ 12.0
Liabilities
Derivative liability	2.0	0.7
Fair value measurements on a recurring basis
Assets
Derivative asset	3.7	12.0
Liabilities
Derivative liability	2.0	0.7
Fair value measurements on a recurring basis | Level 1
Assets
Derivative asset	1.6	6.2
Liabilities
Derivative liability	0.3	0.2
Fair value measurements on a recurring basis | Level 2
Assets
Derivative asset	1.0	2.7
Liabilities
Derivative liability	1.7	0.5
Fair value measurements on a recurring basis | Level 3
Assets
Derivative asset	1.1	3.1
Liabilities
Derivative liability	0.0	0.0
Fair value measurements on a recurring basis | Natural gas contracts
Assets
Derivative asset	1.6	6.8
Liabilities
Derivative liability		0.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative asset	1.6	6.0
Liabilities
Derivative liability		0.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative asset	0.0	0.8
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 | Petroleum products contracts
Assets
Derivative asset		0.2
Liabilities
Derivative liability	0.3	0.1
Fair value measurements on a recurring basis | Petroleum products contracts | Level 1
Assets
Derivative asset		0.2
Liabilities
Derivative liability	0.3	0.1
Fair value measurements on a recurring basis | Petroleum products contracts | Level 2
Assets
Derivative asset		0.0
Liabilities
Derivative liability	0.0	0.0
Fair value measurements on a recurring basis | Petroleum products contracts | Level 3
Assets
Derivative asset		0.0
Liabilities
Derivative liability	0.0	0.0
Fair value measurements on a recurring basis | FTRs
Assets
Derivative asset	1.1	3.1
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	1.1	3.1
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative asset	1.0	1.9
Liabilities
Derivative liability	1.7	0.5
Fair value measurements on a recurring basis | Coal contracts | Level 1
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Fair value measurements on a recurring basis | Coal contracts | Level 2
Assets
Derivative asset	1.0	1.9
Liabilities
Derivative liability	1.7	0.5
Fair value measurements on a recurring basis | Coal contracts | Level 3
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	$ 0.0	$ 0.0