Derivative Instruments (Details) (USD $)	9 Months Ended				3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2012 BillionsCubicFeet	Dec. 31, 2011 BillionsCubicFeet	Sep. 30, 2012 Interest rate derivatives	Dec. 31, 2011 Interest rate derivatives	Sep. 30, 2012 Commodity derivatives designated as cash flow hedges Interest rate derivatives	Sep. 30, 2011 Commodity derivatives designated as cash flow hedges Interest rate derivatives	Sep. 30, 2012 Commodity derivatives designated as cash flow hedges Interest rate derivatives	Sep. 30, 2011 Commodity derivatives designated as cash flow hedges Interest rate derivatives	Sep. 30, 2012 Commodity derivatives designated as cash flow hedges Commodity contracts	Sep. 30, 2011 Commodity derivatives designated as cash flow hedges Commodity contracts	Sep. 30, 2012 Commodity derivatives designated as cash flow hedges Commodity contracts	Sep. 30, 2011 Commodity derivatives designated as cash flow hedges Commodity contracts	Sep. 30, 2012 Commodity derivatives designated as fair value hedges Commodity contracts	Sep. 30, 2011 Commodity derivatives designated as fair value hedges Commodity contracts	Sep. 30, 2012 Commodity derivatives designated as fair value hedges Commodity contracts	Sep. 30, 2011 Commodity derivatives designated as fair value hedges Commodity contracts
Derivatives designated as hedging instruments
Amount of (loss) gain recognized in other comprehensive income (OCI) (effective portion), net of tax					$ (1,800,000)	$ (4,674,000)	$ (6,097,000)	$ (4,674,000)	$ (51,397,000)	$ 69,204,000	$ 47,159,000	$ 106,657,000
Amount of (loss) gain reclassified from accumulated OCI into operating revenues (effective portion), net of tax					(66,000)	(29,000)	(195,000)	(87,000)	45,283,000	15,255,000	148,553,000	42,078,000
Amount of gain (loss) recognized in operating revenues (ineffective portion)									166,000	(352,000)	(76,000)	(613,000)
Amount of (loss) gain recognized in operating revenues for fair value commodity contracts													(3,051,000)	4,261,000	1,644,000	3,728,000
Fair value gain (loss) recognized in operating revenues for inventory designated as hedged item													1,491,000	(3,781,000)	(52,000)	(2,088,000)
Derivatives not designated as hedging instruments:
Amount of (loss) gain recognized in operating revenues									(632,000)	2,663,000	1,041,000	1,840,000
Amount of gain (loss) recognized in income due to exclusion of the spot/forward differential from the assessment of effectiveness													(1,100,000)	800,000	1,600,000	2,300,000
Amount of loss (gain) recognized in income due to changes in basis													500,000	300,000	(100,000)	700,000
Natural Gas Hedging Instruments
Absolute quantities of derivative commodity instruments that have been designated and qualify as cash flow hedges (in Bcf)	361	349
Absolute quantities of the derivative commodity instruments that have been designated and qualify as fair value hedges (in Bcf)	9	9
Deferred losses recorded in accumulated other comprehensive income, net of tax			(13,500,000)	(7,600,000)
Deferred net gains in accumulated other comprehensive income (loss), net of tax	130,700,000	232,100,000
Net unrealized gains on derivative commodity instruments	91,000,000
Margin deposits with brokers	600,000	100,000
Additional collateral limit for below investment grade derivative instruments, as a percentage of the derivative liability	100.00%
Aggregate fair value of derivative instruments with credit-risk related contingencies	8,700,000
Additional collateral for downgrade below investment grade	$ 1,500,000