DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Derivative Asset
Other current derivative assets	$ 15.5	$ 15.3
Other long-term derivative assets	0.3	0.6
Derivative asset	15.8	15.9
Derivative Liability
Other current derivative liabilities	24.5	5.8
Other long-term derivative liabilities	6.6	2.1
Derivative liability	31.1	7.9
Natural gas contracts
Derivative Asset
Other current derivative assets	4.6	7.7
Other long-term derivative assets	0.0	0.4
Derivative Liability
Other current derivative liabilities	22.5	5.3
Other long-term derivative liabilities	1.8	0.2
FTRs
Derivative Asset
Other current derivative assets	10.4	7.4
Derivative Liability
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative Asset
Other current derivative assets	0.5	0.2
Other long-term derivative assets	0.3	0.2
Derivative Liability
Other current derivative liabilities	0.1	0.1
Other long-term derivative liabilities	0.0	0.0
Interest rate swaps
Derivative Asset
Other current derivative assets	0.0	0.0
Other long-term derivative assets	0.0	0.0
Derivative Liability
Other current derivative liabilities	1.9	0.4
Other long-term derivative liabilities	$ 4.8	$ 1.9