Derivative Instruments (Details) (USD $)	9 Months Ended				9 Months Ended	12 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended			12 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Commodity contracts	Dec. 31, 2012 Commodity contracts	Sep. 30, 2013 Commodity derivatives designated as cash flow hedges MMcf	Dec. 31, 2012 Commodity derivatives designated as cash flow hedges MMcf	Sep. 30, 2013 Commodity derivatives designated as cash flow hedges Interest rate swaps settled	Sep. 30, 2012 Commodity derivatives designated as cash flow hedges Interest rate swaps settled	Sep. 30, 2013 Commodity derivatives designated as cash flow hedges Interest rate swaps settled	Sep. 30, 2012 Commodity derivatives designated as cash flow hedges Interest rate swaps settled	Sep. 30, 2013 Commodity derivatives designated as cash flow hedges Commodity contracts	Sep. 30, 2012 Commodity derivatives designated as cash flow hedges Commodity contracts	Sep. 30, 2013 Commodity derivatives designated as cash flow hedges Commodity contracts	Sep. 30, 2012 Commodity derivatives designated as cash flow hedges Commodity contracts	Sep. 30, 2013 Commodity derivatives designated as fair value hedges MMcf	Dec. 31, 2012 Commodity derivatives designated as fair value hedges MMcf	Sep. 30, 2013 Commodity derivatives designated as fair value hedges Commodity contracts	Sep. 30, 2012 Commodity derivatives designated as fair value hedges Commodity contracts	Sep. 30, 2013 Commodity derivatives designated as fair value hedges Commodity contracts	Sep. 30, 2012 Commodity derivatives designated as fair value hedges Commodity contracts
Derivatives designated as hedging instruments
Amount of gain (loss) recognized in OCI (effective portion), net of tax								$ (1,800,000)		$ (6,097,000)	$ 17,733,000	$ (51,397,000)	$ 38,561,000	$ 47,159,000
Amount of gain (loss) reclassified from accumulated OCI into operating revenues (effective portion), net of tax							(36,000)	(66,000)	(108,000)	(195,000)	26,020,000	45,283,000	62,343,000	148,553,000
Amount of gain (loss) recognized in operating revenues (ineffective portion)											3,436,000	166,000	(4,518,000)	(76,000)
Amount of (loss) gain recognized in operating revenues for fair value commodity contracts																	(502,000)	(3,051,000)	(1,341,000)	1,644,000
Fair value (loss) gain recognized in operating revenues for inventory designated as hedged item																	(76,000)	1,491,000	386,000	(52,000)
Derivatives not designated as hedging instruments:
Amount of (loss) gain recognized in operating revenues											(943,000)	(632,000)	307,000	1,041,000
Amount of gain (loss) recognized in income due to exclusion of the spot/forward differential from the assessment of effectiveness																	(1,600,000)	(1,100,000)	500,000	1,600,000
Amount of gain (loss) recognized in income due to changes in basis																	1,000,000	(500,000)	(1,500,000)	100,000
Natural Gas Hedging Instruments
Absolute quantities of derivative commodity instruments that have been designated and qualify as cash flow or fair value hedges (in Bcf)					380,000	365,000									3,000	8,000
Deferred net gains in accumulated other comprehensive income (loss), net of tax	114,400,000	138,200,000
Net unrealized gains on derivative commodity instruments	67,400,000
Margin deposits remitted by brokers with company	0	0
Margin deposits with brokers	400,000	700,000	428,000	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	400,000
Collateral posted	0
Additional collateral for downgrade below investment grade	$ 300,000