Derivative Instruments (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 MMcf	Jun. 30, 2013	Dec. 31, 2013 MMcf
Derivatives designated as hedging instruments
Pre-tax gain recorded in earnings from dedesignation of hedge	$ 28,000,000		$ 28,000,000
Amount of gain reclassified from accumulated OCI, net of tax, into gain on sale / exchange of assets due to forecasted transactions probable to not occur	(16,735,000)		(16,735,000)
Derivatives not designated as hedging instruments:
Amount of (loss) gain recognized in operating revenues			(17,879,000)	1,250,000
Natural Gas Hedging Instruments
Deferred net gains in accumulated other comprehensive income (loss), net of tax	20,500,000		20,500,000		61,700,000
Net unrealized gains on derivative commodity instruments	8,500,000		8,500,000
Margin deposits with brokers	100,000		100,000		300,000
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	31,200,000		31,200,000
Collateral posted	1,100,000		1,100,000
Margin deposits remitted by brokers with company	0		0		0
Commodity derivatives
Natural Gas Hedging Instruments
Margin deposits with brokers	103,000		103,000		266,000
Cash flow hedges | Interest rate derivatives
Derivatives designated as hedging instruments
Amount of (loss) gain reclassified from accumulated OCI into operating revenues/interest expense (effective portion), net of tax	36,000	36,000	72,000	72,000
Cash flow hedges | Commodity derivatives
Derivatives designated as hedging instruments
Amount of (loss) gain recognized in OCI (effective portion), net of tax	(13,455,000)	73,429,000	(52,649,000)	20,828,000
Amount of gain reclassified from accumulated OCI, net of tax, into gain on sale / exchange of assets due to forecasted transactions probable to not occur	16,735,000		16,735,000
Amount of (loss) gain reclassified from accumulated OCI into operating revenues/interest expense (effective portion), net of tax	(10,883,000)	10,490,000	(28,146,000)	36,323,000
Amount of gain (loss) recognized in operating revenues (ineffective portion)	987,000	(7,473,000)	(21,273,000)	(7,954,000)
Amount of gain (loss) recognized in operating revenues for fair value commodity contracts		2,700,000		(839,000)
Fair value (loss) gain recognized in operating revenues for inventory designated as hedged item		(4,075,000)		462,000
Amounts excluded from effectiveness testing of cash flow hedges	0	0	0	0
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)			307,000		398,000
Fair value hedges | Commodity derivatives
Derivatives designated as hedging instruments
Amount of gain (loss) recognized in income due to exclusion of the spot/forward differential from the assessment of effectiveness of fair value hedges		800,000		2,100,000
Amount of gain (loss) recognized in income due to changes in basis		2,200,000		2,500,000
Not designated as hedging instruments
Derivatives not designated as hedging instruments:
Amount of (loss) gain recognized in operating revenues	$ (8,525,000)	$ 1,512,000	$ (17,879,000)	$ 1,250,000