Derivative Instruments (Details) (USD $)	3 Months Ended	6 Months Ended				3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2012 BillionsCubicFeet	Jun. 30, 2012 BillionsCubicFeet	Dec. 31, 2011 BillionsCubicFeet	Jun. 30, 2012 Interest rate derivatives	Dec. 31, 2011 Interest rate derivatives	Jun. 30, 2012 Commodity derivatives designated as cash flow hedges Interest rate derivatives	Jun. 30, 2011 Commodity derivatives designated as cash flow hedges Interest rate derivatives	Jun. 30, 2012 Commodity derivatives designated as cash flow hedges Interest rate derivatives	Jun. 30, 2011 Commodity derivatives designated as cash flow hedges Interest rate derivatives	Jun. 30, 2012 Commodity derivatives designated as cash flow hedges Commodity contracts	Jun. 30, 2011 Commodity derivatives designated as cash flow hedges Commodity contracts	Jun. 30, 2012 Commodity derivatives designated as cash flow hedges Commodity contracts	Jun. 30, 2011 Commodity derivatives designated as cash flow hedges Commodity contracts	Jun. 30, 2012 Commodity derivatives designated as fair value hedges Commodity contracts	Jun. 30, 2011 Commodity derivatives designated as fair value hedges Commodity contracts	Jun. 30, 2012 Commodity derivatives designated as fair value hedges Commodity contracts	Jun. 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						$ (6,670,000)		$ (4,297,000)		$ (8,930,000)	$ 33,253,000	$ 98,558,000	$ 37,453,000
Amount of gain reclassified from accumulated OCI into operating revenues (effective portion), net of tax						(64,000)	(29,000)	(129,000)	(58,000)	55,286,000	9,919,000	103,272,000	26,823,000
Amount of (loss) gain recognized in operating revenues (ineffective portion)										(212,000)	364,000	(243,000)	(261,000)
Amount of gain (loss) recognized in operating revenues for fair value commodity contracts														(2,378,000)	1,363,000	4,694,000	(533,000)
Fair value gain (loss) recognized in operating revenues for inventory designated as hedged item														8,388,000	60,000	(1,543,000)	1,693,000
Derivatives not designated as hedging instruments:
Amount of gain (loss) recognized in operating revenues										(790,000)	856,000	1,673,000	(823,000)
Amount of gain (loss) recognized in income due to exclusion of the spot/forward differential from the assessment of effectiveness														6,000,000	2,100,000	2,800,000	1,500,000
Amount of loss (gain) recognized in income due to changes in basis															700,000	400,000	300,000
Natural Gas Hedging Instruments
Absolute quantities of derivative commodity instruments that have been designated and qualify as cash flow hedges (in Bcf)	343	343	349
Absolute quantities of the derivative commodity instruments that have been designated and qualify as fair value hedges (in Bcf)	10	10	9
Deferred loss recorded in accumulated other comprehensive income, net of tax				11,800,000	7,600,000
Deferred net gains in accumulated other comprehensive income (loss), net of tax	227,400,000	227,400,000	232,100,000
Net unrealized gains on derivative commodity instruments	136,000,000	136,000,000
Margin deposits with brokers	700,000	700,000	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	3,800,000	3,800,000
Additional collateral for downgrade below investment grade	1,700,000	1,700,000
Revenue increase (decrease) related to recognition of put premium	(8,227,000)	(8,227,000)
Accumulated other comprehensive income (decrease) related to recognition of put premium	5,100,000
Tax effect of recognition of put premium	$ (3,100,000)