FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Assets
Derivative Asset	$ 5.5	$ 4.2
Liabilities
Derivative Liability	4.0	1.1
Fair value measurements on a recurring basis
Assets
Derivative Asset	5.5	4.2
Fair value measurements on a recurring basis | Level 1
Assets
Derivative Asset	0.2	0.8
Fair value measurements on a recurring basis | Level 2
Assets
Derivative Asset	0.7	0.4
Fair value measurements on a recurring basis | Level 3
Assets
Derivative Asset	4.6	3.0
Fair value measurements on a recurring basis | Natural gas contracts
Assets
Derivative Asset	0.2	0.8
Liabilities
Derivative Liability	4.0	1.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative Asset	0.2	0.8
Liabilities
Derivative Liability	3.9	1.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative Asset	0.0	0.0
Liabilities
Derivative Liability	0.1	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	0.0
Fair value measurements on a recurring basis | FTRs
Assets
Derivative Asset	4.6	3.0
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	4.6	3.0
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative Asset	0.7	0.4
Fair value measurements on a recurring basis | Coal contracts | Level 1
Assets
Derivative Asset	0.0	0.0
Fair value measurements on a recurring basis | Coal contracts | Level 2
Assets
Derivative Asset	0.7	0.4
Fair value measurements on a recurring basis | Coal contracts | Level 3
Assets
Derivative Asset	$ 0.0	$ 0.0