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, 2011 segments BillionsCubicFeet	Dec. 31, 2010 BillionsCubicFeet	Sep. 30, 2011 Derivatives designated as cash flow hedges Commodity contracts	Sep. 30, 2010 Derivatives designated as cash flow hedges Commodity contracts	Sep. 30, 2011 Derivatives designated as cash flow hedges Commodity contracts	Sep. 30, 2010 Derivatives designated as cash flow hedges Commodity contracts	Sep. 30, 2011 Derivatives designated as fair value hedges Commodity contracts	Sep. 30, 2011 Derivatives designated as fair value hedges Commodity contracts	Sep. 30, 2011 Forward-starting interest rate swap	Sep. 30, 2011 Commodity contracts	Sep. 30, 2010 Commodity contracts	Sep. 30, 2011 Commodity contracts	Sep. 30, 2010 Commodity contracts
Derivatives designated as hedging instruments
Amount of gain (loss) recognized in other comprehensive income (OCI) (effective portion), net of tax			$ 64,530,000	$ 59,120,000	$ 101,982,000	$ 120,346,000
Amount of gain reclassified from accumulated OCI into income (effective portion), net of tax			15,255,000	17,331,000	42,078,000	45,549,000
Amount of gain (loss) recognized in income (ineffective portion)			(352,000)	2,980,000	(613,000)	2,367,000
Amount of gain (loss) recognized in income for fair value commodity contracts							4,261,000	3,728,000
Fair value gain recognized in income for inventory designated as hedged item							(3,781,000)	(2,088,000)
Derivatives not designated as hedging instruments:
Amount of gain (loss) recognized in income										2,663,000	(1,323,000)	1,840,000	(1,234,000)
Unrealized hedge gains reclassified into earnings			2,100,000	7,900,000	2,100,000	10,500,000
Amount of gain recognized in income due to exclusion of the spot/forward differential from the assessment of effectiveness							800,000	2,300,000
Amount of loss recognized in income due to changes in basis							300,000	700,000
Natural Gas Hedging Instruments
Absolute quantities of derivative commodity instruments that have been designated and qualify as cash flow hedges (in Bcf)	221	181
Absolute quantities of the derivative commodity instruments that have been designated and qualify as fair value hedges (in Bcf)	6
Deferred loss recorded in accumulated other comprehensive income, net of tax	125,006,000	65,014,000							(4,700,000)
Deferred net gains in accumulated other comprehensive income (loss), net of tax	129,800,000	65,200,000
Net unrealized gains on derivative commodity instruments	78,600,000
Margin deposits with brokers	1,800,000
Margin deposits remitted by brokers with company		500,000
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	2,300,000
Additional collateral for downgrade below investment grade	$ 2,300,000