RISK MANAGEMENT ACTIVITIES - NOTIONAL VOLUMES (Details) (Non-hedge derivatives)	3 Months Ended	12 Months Ended
	Mar. 31, 2015 MMBTU	Dec. 31, 2013 MMBTU
Natural gas contracts | Purchases
Risk management activities
Notional volume of outstanding derivative contracts (mmbtu or kwh)	66,900,000	186,000,000
FTRs | Other transactions
Risk management activities
Notional volume of outstanding derivative contracts (mmbtu or kwh)	2,111,100,000	4,287,700,000
Petroleum product contracts | Purchases
Risk management activities
Notional volume of outstanding derivative contracts (barrels)	100,000	100,000
Coal contracts | Purchases
Risk management activities
Notional volume of outstanding derivative contracts (tons)	2,600,000	3,000,000