FAIR VALUE - SIGNIFICANT UNOBSERVABLE INPUTS (Details) $ in Millions	6 Months Ended
	Jun. 30, 2015 USD ($) $ / T $ / MWM	Dec. 31, 2014 USD ($)
Significant internally-developed unobservable inputs
Risk Management Assets	$ 7.7	$ 4.5
Risk Management Liabilities	29.5	49.1
Fair value measurements on a recurring basis | Level 3
Significant internally-developed unobservable inputs
Risk Management Assets	4.1	2.2
Risk Management Liabilities	5.4	9.1
Fair value measurements on a recurring basis | Level 3 | FTRs
Significant internally-developed unobservable inputs
Risk Management Assets	4.1	2.2
Risk Management Liabilities	$ 0.0	0.3
Fair value measurements on a recurring basis | Level 3 | FTRs | Valuation Technique: Market-based | Weighted Average
Significant internally-developed unobservable inputs
Forward market prices | $ / MWM	172.71
Fair value measurements on a recurring basis | Level 3 | Coal contracts
Significant internally-developed unobservable inputs
Risk Management Assets	$ 0.0
Risk Management Liabilities	$ 5.4	$ 2.2
Fair value measurements on a recurring basis | Level 3 | Coal contracts | Valuation Technique: Market-based | Minimum
Significant internally-developed unobservable inputs
Forward market prices | $ / T	9.86
Fair value measurements on a recurring basis | Level 3 | Coal contracts | Valuation Technique: Market-based | Maximum
Significant internally-developed unobservable inputs
Forward market prices | $ / T	13.23