Derivative Instruments (Details) (USD $)	3 Months Ended				3 Months Ended
	Mar. 31, 2013 MMcf	Dec. 31, 2012 MMcf	Mar. 31, 2013 Commodity contracts	Dec. 31, 2012 Commodity contracts	Mar. 31, 2013 Commodity derivatives designated as cash flow hedges Interest rate derivatives	Mar. 31, 2012 Commodity derivatives designated as cash flow hedges Interest rate derivatives	Mar. 31, 2013 Commodity derivatives designated as cash flow hedges Commodity contracts	Mar. 31, 2012 Commodity derivatives designated as cash flow hedges Commodity contracts	Mar. 31, 2013 Commodity derivatives designated as fair value hedges Commodity contracts	Mar. 31, 2012 Commodity derivatives designated as fair value hedges Commodity contracts
Derivatives designated as hedging instruments
Amount of (loss) gain recognized in OCI (effective portion), net of tax						$ 2,373,000	$ (52,601,000)	$ 107,488,000
Amount of gain (loss) reclassified from accumulated OCI into operating revenues (effective portion), net of tax					(36,000)	(65,000)	25,833,000	47,986,000
Amount of loss recognized in operating revenues (ineffective portion)							(481,000)	(31,000)
Amount of (loss) gain recognized in operating revenues for fair value commodity contracts									(3,539,000)	7,072,000
Fair value gain (loss) recognized in operating revenues for inventory designated as hedged item									4,537,000	(9,931,000)
Derivatives not designated as hedging instruments:
Amount of (loss) gain recognized in operating revenues							(262,000)	2,463,000
Amount of gain (loss) recognized in income due to exclusion of the spot/forward differential from the assessment of effectiveness									1,300,000	(3,300,000)
Amount of loss (gain) recognized in income due to changes in basis									300,000	(400,000)
Natural Gas Hedging Instruments
Absolute quantities of derivative commodity instruments that have been designated and qualify as cash flow hedges (in Bcf)	428,000	365,000
Absolute quantities of the derivative commodity instruments that have been designated and qualify as fair value hedges (in Bcf)	7,000	8,000
Deferred net gains in accumulated other comprehensive income (loss), net of tax	59,800,000	138,200,000
Net unrealized gains on derivative commodity instruments	30,700,000
Margin deposits remitted by brokers with company	0	0
Margin deposits with brokers	1,100,000	700,000	1,125,000	736,000
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	29,400,000
Collateral posted	0
Additional collateral for downgrade below investment grade	$ 1,600,000