FAIR VALUE - SIGNIFICANT UNOBSERVABLE INPUTS (Details) (USD $) In Millions, unless otherwise specified							3 Months Ended			3 Months Ended						3 Months Ended		3 Months Ended				3 Months Ended				3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Fair value measurements on a recurring basis Level 3	Dec. 31, 2013 Fair value measurements on a recurring basis Level 3	Mar. 31, 2014 Fair value measurements on a recurring basis Utility segments Level 3 FTRs	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Level 3 FTRs	Mar. 31, 2014 Fair value measurements on a recurring basis Utility segments Level 3 FTRs Valuation Technique: Market-based Weighted Average	Mar. 31, 2014 Fair value measurements on a recurring basis Utility segments Level 3 Coal contracts	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Level 3 Coal contracts	Mar. 31, 2014 Fair value measurements on a recurring basis Utility segments Level 3 Coal contracts Valuation Technique: Market-based Minimum	Mar. 31, 2014 Fair value measurements on a recurring basis Utility segments Level 3 Coal contracts Valuation Technique: Market-based Maximum	Mar. 31, 2014 Fair value measurements on a recurring basis Utility segments Level 3 Natural gas contracts	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Level 3 Natural gas contracts	Mar. 31, 2014 Fair value measurements on a recurring basis Nonregulated segments Level 3	Dec. 31, 2013 Fair value measurements on a recurring basis Nonregulated segments Level 3	Mar. 31, 2014 Fair value measurements on a recurring basis Nonregulated segments Level 3 Valuation Technique: Market-based	Mar. 31, 2014 Fair value measurements on a recurring basis Nonregulated segments Level 3 Contingent consideration Maximum	Mar. 31, 2014 Fair value measurements on a recurring basis Nonregulated segments Level 3 Contingent consideration Valuation Technique: Income-based Minimum	Mar. 31, 2014 Fair value measurements on a recurring basis Nonregulated segments Level 3 Contingent consideration Valuation Technique: Income-based Maximum	Mar. 31, 2014 Fair value measurements on a recurring basis Nonregulated segments Level 3 Natural gas contracts	Dec. 31, 2013 Fair value measurements on a recurring basis Nonregulated segments Level 3 Natural gas contracts	Mar. 31, 2014 Fair value measurements on a recurring basis Nonregulated segments Level 3 Natural gas contracts Valuation Technique: Market-based Minimum	Mar. 31, 2014 Fair value measurements on a recurring basis Nonregulated segments Level 3 Natural gas contracts Valuation Technique: Market-based Maximum	Mar. 31, 2014 Fair value measurements on a recurring basis Nonregulated segments Level 3 Electric contracts	Dec. 31, 2013 Fair value measurements on a recurring basis Nonregulated segments Level 3 Electric contracts	Mar. 31, 2014 Fair value measurements on a recurring basis Nonregulated segments Level 3 Electric contracts Valuation Technique: Market-based	Mar. 31, 2014 Fair value measurements on a recurring basis Nonregulated segments Level 3 Electric contracts Valuation Technique: Market-based Minimum	Mar. 31, 2014 Fair value measurements on a recurring basis Nonregulated segments Level 3 Electric contracts Valuation Technique: Market-based Maximum
Significant internally-developed unobservable inputs
Risk Management Assets	$ 330.4	$ 315.5	$ 52.0	$ 53.8	$ 0.9	$ 2.1		$ 1.8	$ 0.2			$ 0	$ 0							$ 27.5	$ 35.6			$ 21.8	$ 15.9
Risk Management Liabilities	222.0	226.6	26.8	31.7	0.2	0.3		1.5	2.7			0	0							22.0	25.2			3.1	3.5
Contingent consideration related to the acquisition of Compass														$ 7.8	$ 7.8		$ 8.0
Forward market prices							172.92			12.11	15.25											(2.59)	3.85				(5.02)	10.06
Probability of default (as a percent)																						11.60%	51.00%			26.00%
Option volatilities (as a percent)																											19.80%	155.60%
Monthly curve shaping (as a percent)																											(27.00%)
Growth rate (as a percent)																		(13.20%)	49.30%
Counterparty default time period																1 year