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Derivative Instruments (Details) (USD $)
 3 Months Ended 3 Months Ended
Mar. 31, 2012
BillionsCubicFeet
Dec. 31, 2011
BillionsCubicFeet
Mar. 31, 2012
Interest rate derivatives
Dec. 31, 2011
Interest rate derivatives
Mar. 31, 2012
Commodity derivatives designated as cash flow hedges
Interest rate derivatives
Mar. 31, 2011
Commodity derivatives designated as cash flow hedges
Interest rate derivatives
Mar. 31, 2012
Commodity derivatives designated as cash flow hedges
Commodity contracts
Mar. 31, 2011
Commodity derivatives designated as cash flow hedges
Commodity contracts
Mar. 31, 2012
Commodity derivatives designated as fair value hedges
Commodity contracts
Mar. 31, 2011
Commodity derivatives designated as fair value hedges
Commodity contracts
Derivatives designated as hedging instruments                    
Amount of gain recognized in other comprehensive income (OCI) (effective portion), net of tax         $ 2,373,000   $ 107,488,000 $ 4,199,000    
Amount of gain reclassified from accumulated OCI into operating revenues (effective portion), net of tax         (65,000) (30,000) 47,986,000 16,905,000    
Amount of loss recognized in operating revenues (ineffective portion)             (31,000) (625,000)    
Amount of gain (loss) recognized in operating revenues for fair value commodity contracts                 7,072,000 (1,896,000)
Fair value gain (loss) recognized in operating revenues for inventory designated as hedged item                 (9,931,000) 1,633,000
Derivatives not designated as hedging instruments:                    
Amount of gain (loss) recognized in operating revenues             2,463,000 (1,679,000)    
Amount of loss recognized in income due to exclusion of the spot/forward differential from the assessment of effectiveness                 3,300,000  
Amount of gain recognized in income due to changes in basis                 400,000  
Natural Gas Hedging Instruments                    
Absolute quantities of derivative commodity instruments that have been designated and qualify as cash flow hedges (in Bcf) 316 349                
Absolute quantities of the derivative commodity instruments that have been designated and qualify as fair value hedges (in Bcf) 11 9                
Deferred loss recorded in accumulated other comprehensive income, net of tax     5,200,000 7,600,000            
Deferred net gains in accumulated other comprehensive income (loss), net of tax 291,600,000 232,100,000                
Net unrealized gains on derivative commodity instruments 186,600,000                  
Margin deposits with brokers 1,300,000 100,000                
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 2,000,000                  
Additional collateral for downgrade below investment grade $ 2,000,000