Fair Value Measurements (Details) (USD $)	Dec. 31, 2010	Dec. 31, 2011 3.50% Senior Convertible Notes due 2027	Dec. 31, 2010 3.50% Senior Convertible Notes due 2027	Dec. 31, 2011 6.625% Senior Notes due 2019	Feb. 07, 2011 6.625% Senior Notes due 2019	Dec. 31, 2011 6.50% Senior Notes due 2021	Nov. 03, 2011 6.50% Senior Notes due 2021	Dec. 31, 2011 Recurring basis Level 1	Dec. 31, 2010 Recurring basis Level 1	Dec. 31, 2010 Recurring basis Level 1 Derivatives designated as hedging instruments	Dec. 31, 2011 Recurring basis Level 1 Derivatives not designated as hedging instruments	Dec. 31, 2011 Recurring basis Level 2	Dec. 31, 2010 Recurring basis Level 2 Derivatives designated as hedging instruments	Dec. 31, 2011 Recurring basis Level 2 Derivatives not designated as hedging instruments	Dec. 31, 2011 Recurring basis Level 3	Dec. 31, 2010 Recurring basis Level 3	Dec. 31, 2010 Recurring basis Level 3 Derivatives designated as hedging instruments	Dec. 31, 2011 Recurring basis Level 3 Derivatives not designated as hedging instruments	Dec. 31, 2011 Fair Value, Measurements, Nonrecurring [Member] Level 1	Dec. 31, 2011 Fair Value, Measurements, Nonrecurring [Member] Level 2	Dec. 31, 2011 Fair Value, Measurements, Nonrecurring [Member] Level 3
Assets:
Derivatives										$ 0	$ 0		$ 62,332,000	$ 86,875,000			$ 0	$ 0
Proved oil and gas properties																			0	0	139,992,000
Unproved Oil and Gas Property, Successful Efforts Method, Fair Value																			0	0	15,809,000
Liabilities:										0	0		114,601,000	55,681,000			0	0
Net Profits Plan									0							0
Interest rate on notes (as a percent)		3.50%	3.50%	6.625%	6.625%	6.50%	6.50%
Approximate estimated fair value of notes		394,100,000	351,000,000	359,400,000		360,300,000
Net Profits Plan liability	$ 135,850,000							$ 0				$ 0			$ 107,731,000