DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivative Asset
Other current derivative assets	$ 9.2	$ 15.3
Other long-term derivative assets	1.1	0.6
Derivative asset	10.3	15.9
Derivative Liability
Other current derivative liabilities	1.7	5.8
Other long-term derivative liabilities	3.0	2.1
Derivative liability	4.7	7.9
Natural gas contracts
Derivative Asset
Other current derivative assets	5.1	7.7
Other long-term derivative assets	0.9	0.4
Derivative Liability
Other current derivative liabilities	0.9	5.3
Other long-term derivative liabilities	0.0	0.2
FTRs
Derivative Asset
Other current derivative assets	3.1	7.4
Derivative Liability
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative Asset
Other current derivative assets	1.0	0.2
Other long-term derivative assets	0.2	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	0.7	0.4
Other long-term derivative liabilities	$ 3.0	$ 1.9