DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Derivative Asset [Abstract]
Other current derivative assets	$ 8.2	$ 2.3
Other long-term derivative assets	0.0	0.0
Derivative Asset	8.2	2.3
Derivative Liability [Abstract]
Other current derivative liabilities	2.9	4.7
Other long-term derivative liabilities	1.3	1.4
Derivative Liability	4.2	6.1
Natural gas contracts
Derivative Asset [Abstract]
Other current derivative assets	2.3	0.3
Derivative Liability [Abstract]
Other current derivative liabilities	0.1	0.9
Petroleum products contracts
Derivative Asset [Abstract]
Other current derivative assets	0.0	0.0
Derivative Liability [Abstract]
Other current derivative liabilities	0.1	0.5
FTRs
Derivative Asset [Abstract]
Other current derivative assets	5.9	2.0
Derivative Liability [Abstract]
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative Asset [Abstract]
Other current derivative assets	0.0	0.0
Other long-term derivative assets	0.0	0.0
Derivative Liability [Abstract]
Other current derivative liabilities	2.7	3.3
Other long-term derivative liabilities	$ 1.3	$ 1.4