Fair Value Measurements (Details) (USD $)	Sep. 30, 2012		Dec. 31, 2011
Assets:
Proved Oil and Gas Property, Successful Effort Method	$ 5,197,761,000		$ 4,378,987,000
Unproved Oil and Gas Property, Successful Effort Method	160,468,000		120,966,000
Convertible Debt [Member]
Liabilities [Abstract]
Interest rate on notes (as a percent)	3.50%		3.50%
Approximate estimated fair value of notes			394,000,000
6.625% Senior Notes [Member]
Liabilities [Abstract]
Interest rate on notes (as a percent)	6.625%		6.625%
Approximate estimated fair value of notes	371,000,000		359,000,000
6.50% Senior Notes [Member]
Liabilities [Abstract]
Interest rate on notes (as a percent)	6.50%		6.50%
Approximate estimated fair value of notes	370,000,000		360,000,000
6.50% Senior Notes due 2023 [Member]
Liabilities [Abstract]
Interest rate on notes (as a percent)	6.50%
Approximate estimated fair value of notes	422,000,000
Fair Value, Measurements, Recurring [Member] | Level 1
Assets:
Proved Oil and Gas Property, Successful Effort Method			0	[1]
Unproved Oil and Gas Property, Successful Effort Method			0	[1]
Liabilities [Abstract]
Net Profits Plan Liability	0	[2]	0	[2]
Fair Value, Measurements, Recurring [Member] | Level 1 | Derivatives not designated as hedging instruments
Assets:
Derivatives	0	[2]	0	[2]
Liabilities [Abstract]
Derivatives	0	[2]	0	[2]
Fair Value, Measurements, Recurring [Member] | Level 2
Assets:
Proved Oil and Gas Property, Successful Effort Method	0	[1]	0	[1]
Unproved Oil and Gas Property, Successful Effort Method	0	[1]	0	[1]
Liabilities [Abstract]
Net Profits Plan Liability	0	[2]	0	[2]
Fair Value, Measurements, Recurring [Member] | Level 2 | Derivatives not designated as hedging instruments
Assets:
Derivatives	64,248,000	[2]	86,875,000	[2]
Liabilities [Abstract]
Derivatives	28,154,000	[2]	55,681,000	[2]
Fair Value, Measurements, Recurring [Member] | Level 3
Assets:
Proved Oil and Gas Property, Successful Effort Method	2,057,000	[1]	139,992,000	[1]
Unproved Oil and Gas Property, Successful Effort Method	9,200,000	[1]	15,809,000	[1]
Liabilities [Abstract]
Net Profits Plan Liability	90,389,000	[2]	107,731,000	[2]
Fair Value, Measurements, Recurring [Member] | Level 3 | Derivatives not designated as hedging instruments
Assets:
Derivatives	0	[2]	0	[2]
Liabilities [Abstract]
Derivatives	0	[2]	0	[2]
Fair Value, Measurements, Nonrecurring [Member] | Level 1
Assets:
Proved Oil and Gas Property, Successful Effort Method	0	[1]
Unproved Oil and Gas Property, Successful Effort Method	$ 0	[1]

[1] This represents a non-financial asset or liability that is measured at fair value on a nonrecurring basis.

[2] This represents a financial asset or liability that is measured at fair value on a recurring basis.