Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Assets:
Proved oil and gas properties	$ 5,414,842		$ 5,401,684
Unproved oil and gas properties	263,662		175,287
Oil and gas properties held for sale net of accumulated depletion, depreciation and amortization of $539,769 in 2013 and 20,676 in 2012	400,393		33,620
Liabilities:
Asset Retirement Obligations, Noncurrent	101,650		112,912
Fair Value, Measurements, Nonrecurring [Member] | Level 1
Assets:
Proved oil and gas properties	0	[1]	0	[1]
Unproved oil and gas properties	0	[1]	0	[1]
Oil and gas properties held for sale net of accumulated depletion, depreciation and amortization of $539,769 in 2013 and 20,676 in 2012	0	[1]	0	[1]
Liabilities:
Asset Retirement Obligations, Noncurrent	0	[1]
Fair Value, Measurements, Nonrecurring [Member] | Level 2
Assets:
Proved oil and gas properties	0	[1]	0	[1]
Unproved oil and gas properties	0	[1]	0	[1]
Oil and gas properties held for sale net of accumulated depletion, depreciation and amortization of $539,769 in 2013 and 20,676 in 2012	0	[1]	0	[1]
Liabilities:
Asset Retirement Obligations, Noncurrent	0	[1]
Fair Value, Measurements, Nonrecurring [Member] | Level 3
Assets:
Proved oil and gas properties	11,443	[1]	209,959	[1]
Unproved oil and gas properties	20,915	[1]	42,765	[1]
Oil and gas properties held for sale net of accumulated depletion, depreciation and amortization of $539,769 in 2013 and 20,676 in 2012	15,349	[1]	16,527	[1]
Liabilities:
Asset Retirement Obligations, Noncurrent	1,573	[1]
Fair Value, Measurements, Recurring [Member] | Level 1
Liabilities:
Net Profits Plan Liability	0	[2]	0	[2]
Fair Value, Measurements, Recurring [Member] | Level 2
Liabilities:
Net Profits Plan Liability	0	[2]	0	[2]
Fair Value, Measurements, Recurring [Member] | Level 3
Liabilities:
Net Profits Plan Liability	72,404	[2]	78,827	[2]
Derivatives not designated as hedging instruments | Fair Value, Measurements, Recurring [Member] | Level 1
Assets:
Derivatives	0	[2]	0	[2]
Liabilities:
Derivatives	0	[2]	0	[2]
Derivatives not designated as hedging instruments | Fair Value, Measurements, Recurring [Member] | Level 2
Assets:
Derivatives	71,964	[2]	54,339	[2]
Liabilities:
Derivatives	29,521	[2]	15,644	[2]
Derivatives not designated as hedging instruments | Fair Value, Measurements, Recurring [Member] | Level 3
Assets:
Derivatives	0	[2]	0	[2]
Liabilities:
Derivatives	$ 0	[2]	$ 0	[2]

[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.