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

6 Months Ended

Jun. 30, 2011

Jun. 30, 2010

Accumulated other comprehensive income, net gain related to hedging activities

$ 120,100,000

Deferred income tax liability, accumulated other comprehensive income

76,800,000

Cash flow hedge after-tax net gain to be transferred from accumulated other comprehensive income to earnings during the next twelve months

104,700,000

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

$ 105,600,000

$ 112,800,000

Fair value hedges

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 June 30, 2011 and December 31, 2010, the Company had no material fair value hedges.

Year 2011 [Member]

Volume of Natural Gas Production (in Bcf), Basis Swaps Not Qualifying for Hedge Accounting Treatment

16.6

Year 2012 [Member]

Volume of Natural Gas Production (in Bcf), Basis Swaps Not Qualifying for Hedge Accounting Treatment

26.7

Year 2013 [Member]

Volume of Natural Gas Production (in Bcf), Basis Swaps Not Qualifying for Hedge Accounting Treatment

19.1