RISK MANAGEMENT ACTIVITIES (Details 4) (Utility Segments, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Natural gas contracts Balance Sheet - Regulatory assets (current)	Mar. 31, 2012 Natural gas contracts Balance Sheet - Regulatory assets (current)	Mar. 31, 2013 Natural gas contracts Balance Sheet - Regulatory assets (long-term)	Mar. 31, 2012 Natural gas contracts Balance Sheet - Regulatory assets (long-term)	Mar. 31, 2013 Natural gas contracts Balance Sheet - Regulatory liabilities (current)	Mar. 31, 2012 Natural gas contracts Balance Sheet - Regulatory liabilities (current)	Mar. 31, 2013 Natural gas contracts Balance Sheet - Regulatory liabilities (long-term)	Mar. 31, 2012 Natural gas contracts Balance Sheet - Regulatory liabilities (long-term)	Mar. 31, 2012 Natural gas contracts Income Statement - Utility cost of fuel, natural gas and purchased power	Mar. 31, 2013 Natural gas contracts Income Statement - Operating and maintenance expense	Mar. 31, 2013 FTRs Balance Sheet - Regulatory assets (current)	Mar. 31, 2012 FTRs Balance Sheet - Regulatory assets (current)	Mar. 31, 2013 FTRs Balance Sheet - Regulatory liabilities (current)	Mar. 31, 2012 FTRs Balance Sheet - Regulatory liabilities (current)	Mar. 31, 2012 Petroleum product contracts Balance Sheet - Regulatory assets (current)	Mar. 31, 2012 Petroleum product contracts Balance Sheet - Regulatory liabilities (current)	Mar. 31, 2012 Petroleum product contracts Income Statement - Operating and maintenance expense	Mar. 31, 2013 Coal contract Balance Sheet - Regulatory assets (current)	Mar. 31, 2012 Coal contract Balance Sheet - Regulatory assets (current)	Mar. 31, 2013 Coal contract Balance Sheet - Regulatory assets (long-term)	Mar. 31, 2012 Coal contract Balance Sheet - Regulatory assets (long-term)	Mar. 31, 2013 Coal contract Balance Sheet - Regulatory liabilities (current)	Mar. 31, 2013 Coal contract Balance Sheet - Regulatory liabilities (long-term)	Mar. 31, 2013 Purchases Natural gas contracts therm	Dec. 31, 2012 Purchases Natural gas contracts therm	Mar. 31, 2013 Purchases Petroleum product contracts bbl	Dec. 31, 2012 Purchases Petroleum product contracts bbl	Mar. 31, 2013 Purchases Coal contract T	Dec. 31, 2012 Purchases Coal contract T	Dec. 31, 2012 Sales Natural gas contracts therm	Mar. 31, 2013 Sales Coal contract T	Mar. 31, 2013 Other transactions FTRs kWh	Dec. 31, 2012 Other transactions FTRs kWh
Risk management activities
Notional volume of outstanding non-hedge derivative contracts																								595,400,000	1,072,600,000					100,000
Notional volume of outstanding non-hedge derivative contracts																																1,579,000,000	4,057,200,000
Notional volume of outstanding non-hedge derivative contracts																										50,855.0	62,811.0
Notional volume of outstanding non-hedge derivative contracts																												4,700,000	5,100,000		200,000
Unrealized gains (losses) recorded related to non-hedge derivatives	$ 13.0	$ (6.4)	$ 0.8	$ (0.8)	$ 5.9	$ (3.7)	$ 0.8	$ 0.1	$ 0.1	$ 0.2	$ 0.2	$ 0.4	$ (0.4)	$ (0.3)	$ 0.1	$ 0.1	$ 0.1	$ 1.9	$ (3.1)	$ 2.3	$ (3.5)	$ (0.2)	$ (2.2)