Derivatives and Risk Management (Narrative) (Details) (USD $)

12 Months Ended

Dec. 31, 2011

Dec. 31, 2010

Accumulated other comprehensive income, net gain related to hedging activities

$ 424,900,000

Deferred income tax liability, accumulated other comprehensive income

276,200,000

Cash Flow Hedge After Tax Net Gain To Be Transfered From Accumulated Other Comprehensive Income To Earnings During Next Twelve Months

303,200,000

Realized gain from settled contracts

320,100,000

301,500,000

Description of Fair Value Hedge Activity

For fair value hedges, the gain or loss on the derivative instrument as well as the offsetting gain or loss on the hedged item are recognized in earnings immediately. As of December 31, 2011 and December 31, 2010, the Company had no material fair value hedges.

Year 2012 [Member]

Volume of natural gas production (in Bcf), basis swaps not qualifying for hedge accounting treatment

36.6

Year 2013 [Member]

Volume of natural gas production (in Bcf), basis swaps not qualifying for hedge accounting treatment

30.1

Year 2014 [Member]

Volume of natural gas production (in Bcf), basis swaps not qualifying for hedge accounting treatment

9.1

Fixed Price Swaps [Member]

Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion

256,229,000

230,707,000

Costless Collar [Member]

Derivative Instruments, Gain Reclassified from Accumulated Other Comprehensive Income into Earnings, Effective Portion

$ 65,047,000

$ 70,775,000