DERIVATIVE FINANCIAL INSTRUMENTS (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Effect of Cash Flow Hedges on Statement of Income
Gain or (Loss) Reclassified from Accumulated OCI into Income, net of tax (Effective Portion)	$ (7.0)	$ (12.3)	$ (66.9)
Hedge Ineffectiveness
Other gains (losses) recognized in income for ineffectiveness on interest rate hedges designated as cash flow hedges	(1.1)
Credit Risk Considerations
Collateral posted related to derivatives with contingent provisions that are in net liability position	140.3	20.0
Additional collateral required to be posted to counterparties if all underlying contingent features were fully triggered	50.7	74.0
Aggregate fair value of all derivative instruments with contingent provisions that are in a net liability position	191.0	94.0
Commodity contracts
Effect of Cash Flow Hedges on Statement of Income
Derivatives not designated as hedging instruments - gain (loss) recognized in income, classified in gas purchased for resale	(2)	(3)	(16)
Derivatives in Fair Value Hedging Relationships
Effect of Fair Value Hedges on Statement of Income
Reduction to interest expense for combined amount of gains on fair value hedges, losses on hedged fixed rate debt, and amortization of deferred gain on previously terminated swaps	5.8	11.5
Derivatives in Cash Flow Hedging Relationships
Effect of Cash Flow Hedges on Statement of Income
Gain or (Loss) Recognized in OCI, net of tax (Effective Portion)	(58)	(36)	(30)
Gain or (Loss) Reclassified from Accumulated OCI into Income, net of tax (Effective Portion)	(13.0)	(17.0)	(70.0)
Derivatives in Cash Flow Hedging Relationships | Interest rate contracts
Effect of Cash Flow Hedges on Statement of Income
Gain or (Loss) Deferred in Regulatory Accounts (Effective Portion), classified in interest expense	(76)	(36)	42
Gain or (Loss) Reclassified from Deferred Accounts into Income (Effective Portion), classified in interest expense	(3)	(2)	(3)
Gain or (Loss) Recognized in OCI, net of tax (Effective Portion)	(42)	(24)	9
Gain or (Loss) Reclassified from Accumulated OCI into Income, net of tax (Effective Portion)	(4.0)	(4.0)	(3.0)
Expected reclassifications from accumulated other comprehensive loss to earnings arising from cash flow hedges during next 12 months	4.3
Derivatives in Cash Flow Hedging Relationships | Commodity contracts
Effect of Cash Flow Hedges on Statement of Income
Gain or (Loss) Recognized in OCI, net of tax (Effective Portion)	(16)	(12)	(39)
Gain or (Loss) Reclassified from Accumulated OCI into Income, net of tax (Effective Portion)	(9.0)	(13.0)	(67.0)
Expected reclassifications from accumulated other comprehensive loss to earnings arising from cash flow hedges during next 12 months	$ 6.7