DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) $ in Millions	Jun. 30, 2019 USD ($) Instruments	Dec. 31, 2018 USD ($)
Derivative asset
Other current derivative assets	$ 5.2	$ 4.0
Other long-term derivative assets	0.3	0.2
Derivative Asset	5.5	4.2
Derivative liability
Other current derivative liabilities	3.7	1.0
Other long-term derivative liabilities	0.3	0.1
Derivative Liability	4.0	1.1
Natural gas contracts
Derivative asset
Other current derivative assets	0.2	0.8
Other long-term derivative assets	0.0	0.0
Derivative liability
Other current derivative liabilities	3.7	1.0
Other long-term derivative liabilities	0.3	0.1
FTRs
Derivative asset
Other current derivative assets	4.6	3.0
Derivative liability
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative asset
Other current derivative assets	0.4	0.2
Other long-term derivative assets	0.3	0.2
Derivative liability
Other current derivative liabilities	0.0	0.0
Other long-term derivative liabilities	$ 0.0	$ 0.0
Designated as hedging instrument
Derivative assets and liabilities
Number of derivative instruments held | Instruments	0