Derivative Instruments (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Natural Gas Hedging Instruments
Absolute quantities of derivative commodity instruments that have been designated and qualify as cash flow hedges (in Bcf)	349	181
Deferred loss recorded in accumulated other comprehensive income, net of tax	$ 232,066,000	$ 65,226,000
Deferred net gains in accumulated other comprehensive income (loss), net of tax	232,100,000	65,200,000
Net unrealized gains on derivative commodity instruments	150,800,000
Margin deposits with brokers		500,000
Margin deposits remitted by brokers with company	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	2,200,000
Additional collateral for downgrade below investment grade	2,200,000
Interest rate derivatives
Natural Gas Hedging Instruments
Deferred loss recorded in accumulated other comprehensive income, net of tax	7,600,000
Commodity derivatives designated as cash flow hedges | Interest rate derivatives
Derivatives designated as hedging instruments
Amount of gain (loss) recognized in other comprehensive income (OCI) (effective portion), net of tax	(7,573,000)
Amount of gain reclassified from accumulated OCI into income (effective portion), net of tax	(140,000)	(116,000)	(115,000)
Commodity derivatives designated as cash flow hedges | Commodity contracts
Derivatives designated as hedging instruments
Amount of gain (loss) recognized in other comprehensive income (OCI) (effective portion), net of tax	239,019,000	113,320,000	148,327,000
Amount of gain reclassified from accumulated OCI into income (effective portion), net of tax	72,179,000	63,719,000	103,926,000
Amount of gain (loss) recognized in income (ineffective portion)	(181,000)	3,046,000	(2,068,000)
Derivatives not designated as hedging instruments:
Amount of gain recognized in operating revenues	4,209,000	369,000	65,000
Commodity derivatives designated as fair value hedges | Commodity contracts
Derivatives designated as hedging instruments
Amount of gain (loss) recognized in income for fair value commodity contracts	12,263,000
Fair value gain recognized in income for inventory designated as hedged item	(6,059,000)
Derivatives not designated as hedging instruments:
Amount of gain recognized in income due to exclusion of the spot/forward differential from the assessment of effectiveness	7,600,000
Amount of loss recognized in income due to changes in basis	$ 1,400,000