RISK MANAGEMENT ACTIVITIES - NONREGULATED SEGMENTS, GAINS AND LOSSES (Details) (Nonregulated segments, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Risk management activities
Gains (losses) recorded related to non-hedge derivatives (total)	$ 157.0	$ 64.7
Natural gas contracts | Nonregulated revenue
Risk management activities
Gains (losses) recorded related to non-hedge derivatives	(36.9)	3.4
Natural gas contracts | Nonregulated cost of sales
Risk management activities
Gains (losses) recorded related to non-hedge derivatives	33.0	(1.6)
Natural gas contracts | Nonregulated revenue (reclassified from accumulated OCI)
Risk management activities
Gains (losses) recorded related to non-hedge derivatives (prior cash flow hedge)	0	(0.1)
Electric contracts | Nonregulated revenue
Risk management activities
Gains (losses) recorded related to non-hedge derivatives	160.3	64.0
Electric contracts | Nonregulated cost of sales
Risk management activities
Gains (losses) recorded related to non-hedge derivatives	0.6	0
Electric contracts | Nonregulated revenue (reclassified from accumulated OCI)
Risk management activities
Gains (losses) recorded related to non-hedge derivatives (prior cash flow hedge)	$ 0	$ (1.0)