DERIVATIVE FINANCIAL INSTRUMENTS (Details 3) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
DERIVATIVE FINANCIAL INSTRUMENTS
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	$ 0.9	$ 2.5	$ 4.9	$ 8.4
Gain (Loss) on Derivatives
Gain (Loss) Reclassified from Accumulated OCI into Income, net of tax (Effective Portion)	(2)	(3)	(8)	(12)
Credit Risk Considerations
Collateral posted related to derivatives with contingent provisions that are in net liability position	110		110		20
Additional collateral required to be posted to counterparties if all underlying contingent features were fully triggered	58.3		58.3		74.0
Aggregate fair value of all derivative instruments with contingent provisions that are in a net liability position	168.0		168.0		94.0
Hedge Ineffectiveness
Other losses recognized in income for ineffectiveness on interest rate hedges designated as cash flow hedges	(0.8)	(0.1)	(1.1)	(0.2)
Derivatives designated as hedging instruments | Interest rate contracts | Derivatives in Cash Flow Hedging Relationships
Gain (Loss) on Derivatives
Gain (Loss) Deferred in Regulatory Accounts (Effective Portion), classified in interest expense	(63)	(36)	(72)	(96)
Gain (Loss) Reclassified from Deferred Accounts into Income (Effective Portion), classified in interest expense	(1)	(1)	(2)	(2)
Gain (Loss) Recognized in OCI, net of tax (Effective Portion)	(28)	(20)	(39)	(53)
Gain (Loss) Reclassified from Accumulated OCI into Income, net of tax (Effective Portion)	(1)	(1)	(3)	(3)
Expected reclassifications from accumulated other comprehensive loss to earnings arising from cash flow hedges during next 12 months			3.4
Derivatives designated as hedging instruments | Commodity contracts | Derivatives in Cash Flow Hedging Relationships
Gain (Loss) on Derivatives
Gain (Loss) Recognized in OCI, net of tax (Effective Portion)	(5)	(6)	(7)	(13)
Gain (Loss) Reclassified from Accumulated OCI into Income, net of tax (Effective Portion)	(1)	(2)	(5)	(9)
Expected reclassifications from accumulated other comprehensive loss to earnings arising from cash flow hedges during next 12 months			5.2
Derivatives designated as hedging instruments | Derivatives in Cash Flow Hedging Relationships
Gain (Loss) on Derivatives
Gain (Loss) Recognized in OCI, net of tax (Effective Portion)	(33)	(26)	(46)	(66)
Gain (Loss) Reclassified from Accumulated OCI into Income, net of tax (Effective Portion)	(2)	(3)	(8)	(12)
Derivatives not designated as hedging instruments | Commodity contracts
Gain (Loss) on Derivatives
Gain (Loss) Recognized in Income, classified in gas purchased for resale			$ (1)	$ (2)