Fair Value Measurements (Details) (USD $)			3 Months Ended		9 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2011 Net Profit Plan liability [Member]	Mar. 31, 2011 Net Profit Plan liability [Member]	Sep. 30, 2011 Net Profit Plan liability [Member]	Sep. 30, 2011 Net Profits Plan [Member]	Sep. 30, 2010 Net Profits Plan [Member]	Sep. 30, 2011 Net Profits Plan [Member]	Sep. 30, 2010 Net Profits Plan [Member]	Dec. 31, 2010 Derivatives designated as hedging instruments Recurring basis Level 1	Dec. 31, 2010 Derivatives designated as hedging instruments Recurring basis Level 2	Dec. 31, 2010 Derivatives designated as hedging instruments Recurring basis Level 3	Sep. 30, 2011 Derivatives not designated as hedging instruments Recurring basis Level 1	Sep. 30, 2011 Derivatives not designated as hedging instruments Recurring basis Level 2	Sep. 30, 2011 Derivatives not designated as hedging instruments Recurring basis Level 3	Sep. 30, 2011 Recurring basis Level 1	Dec. 31, 2010 Recurring basis Level 1	Sep. 30, 2011 Recurring basis Level 2	Dec. 31, 2010 Recurring basis Level 2	Sep. 30, 2011 Recurring basis Level 3	Dec. 31, 2010 Recurring basis Level 3	Sep. 30, 2011 Derivatives not designated as hedging instruments Fair Value, Measurements, Nonrecurring [Member] Level 1	Sep. 30, 2011 Derivatives not designated as hedging instruments Fair Value, Measurements, Nonrecurring [Member] Level 2	Sep. 30, 2011 Derivatives not designated as hedging instruments Fair Value, Measurements, Nonrecurring [Member] Level 3	Dec. 31, 2010 Derivatives designated as hedging instruments	Sep. 30, 2011 3.50% Senior Convertible Notes due 2027	Dec. 31, 2010 3.50% Senior Convertible Notes due 2027	Sep. 30, 2011 6.625% Senior Notes due 2019	Dec. 31, 2010 6.625% Senior Notes due 2019
Financial assets and liabilities measured at fair value on a recurring basis
Cash payments made under the Net Profits Plan related to divested properties						$ 0	$ 686,000	$ 6,300,000	$ 20,800,000
Amount paid for settlement of liabilities			0	2,600,000	0
Assets:
Derivatives										0	62,332,000	0	0	94,589,000	0										62,332,000
Proved oil and gas properties		0																				0	0	19,113,000
Liabilities:
Derivatives										0	114,601,000	0		24,290,000											114,601,000
Net Profits Plan																	0		0		135,850,000
Interest rate on notes (as a percent)																										3.50%	3.50%	6.625%	6.625%
Approximate estimated fair value of notes																										357,200,000	351,000,000	351,800,000
Embedded contingent interest derivative on debt																											0
Net Profits Plan liability	$ 108,489,000	$ 135,850,000														$ 0		$ 0		$ 108,489,000