DERIVATIVE FINANCIAL INSTRUMENTS (Details 3) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
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	$ 0	$ (2,500,000)
Effect of Cash Flow Hedges on Statement of Income
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	(10,000,000)	(5,000,000)
Gain or (Loss) Deferred in Regulatory Accounts (Effective Portion), classified in interest expense	(4,000,000)	0
Derivative, Credit Risk Related Contingent Features [Abstract]
Collateral Already Posted, Aggregate Fair Value	94,000,000		140,300,000
Additional collateral required to be posted to counterparties if all underlying contingent features were fully triggered	25,500,000		50,700,000
Aggregate fair value of all derivative instruments with contingent provisions that are in a net liability position	119,500,000		0
Commodity contracts
Effect of Cash Flow Hedges on Statement of Income
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	8,000,000	4,000,000
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred	4,900,000
Gain or (Loss) Deferred in Regulatory Accounts (Effective Portion), classified in interest expense	4,000,000	0
Derivatives not designated as hedging instruments - gain (loss) recognized in income, classified in gas purchased for resale	(1,000,000)	(1)
Interest Rate Contract
Gain (Loss) on Derivatives
Derivative Instruments, Gain (Loss) Deferred in Regulatory Accounts Effective Portion, Net	30,000,000	0
Effect of Cash Flow Hedges on Statement of Income
Gain or (Loss) Deferred in Regulatory Accounts (Effective Portion), classified in interest expense	(1,000,000)	(1,000,000)
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	2,000,000	1,000,000
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred	5,800,000
Gain or (Loss) Deferred in Regulatory Accounts (Effective Portion), classified in interest expense	$ 0	$ 0