Fair Value Measurements (Details) (USD $)	Dec. 31, 2012		Dec. 31, 2011
Assets:
Proved oil and gas properties	$ 5,401,684,000		$ 4,378,987,000
Unproved Oil and Gas Property, Successful Efforts Method, Fair Value	175,287,000		120,966,000
Liabilities [Abstract]
Net Profits Plan liability	78,827,000		107,731,000
Recurring basis | Level 1
Liabilities [Abstract]
Net Profits Plan liability	0	[1]	0	[1]
Recurring basis | Level 1 | Not Designated as Hedging Instrument [Member]
Assets:
Derivatives	0	[1]	0	[1]
Liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	[1]	0	[1]
Recurring basis | Level 2
Liabilities [Abstract]
Net Profits Plan liability	0	[1]	0	[1]
Recurring basis | Level 2 | Not Designated as Hedging Instrument [Member]
Assets:
Derivatives	54,339,000	[1]	86,875,000	[1]
Liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	15,644,000	[1]	55,681,000	[1]
Recurring basis | Fair Value, Inputs, Level 3 [Member]
Liabilities [Abstract]
Net Profits Plan liability	78,827,000	[1]	107,731,000	[1]
Recurring basis | Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member]
Assets:
Derivatives	0	[1]	0	[1]
Liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	[1]	0	[1]
Fair Value, Measurements, Nonrecurring [Member] | Level 1
Assets:
Proved oil and gas properties	0	[2]	0	[2]
Unproved Oil and Gas Property, Successful Efforts Method, Fair Value	0	[2]	0	[2]
Fair Value, Measurements, Nonrecurring [Member] | Level 2
Assets:
Proved oil and gas properties	0	[2]	0	[2]
Unproved Oil and Gas Property, Successful Efforts Method, Fair Value	0	[2]	0	[2]
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets:
Proved oil and gas properties	209,959,000	[2]	139,992,000	[2]
Unproved Oil and Gas Property, Successful Efforts Method, Fair Value	$ 42,765,000	[2]	$ 15,809,000	[2]

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

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