Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Assets:
Proved oil and gas properties	$ 6,151,765		$ 5,637,462
Unproved oil and gas properties	388,336		271,100
Oil and gas properties held for sale net of accumulated depletion, depreciation and amortization of $23,697 and $7,390, respectively	23,935		19,072
Liabilities:
Line of Credit Facility, Amount Outstanding	0		0
Asset Retirement Obligations, Noncurrent	117,916		115,659
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 $23,697 and $7,390, respectively	0	[1]	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 $23,697 and $7,390, respectively	0	[1]	0	[1]
Fair Value, Measurements, Nonrecurring [Member] | Level 3
Assets:
Proved oil and gas properties	2,527	[1]	62,178	[1]
Unproved oil and gas properties	3,636	[1]	3,280	[1]
Oil and gas properties held for sale net of accumulated depletion, depreciation and amortization of $23,697 and $7,390, respectively	6,466	[1]	650	[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	48,104	[2]	56,985	[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	4,913	[2]	52,510	[2]
Liabilities:
Derivatives	144,935	[2]	31,020	[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.