Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Assets:
Derivatives	$ 63,805		$ 52,510
Proved oil and gas properties	6,789,927		5,637,462
Unproved oil and gas properties	536,100		271,100
Disposal Group, Including Discontinued Operation, Long Lived Assets, Current	19,900
Liabilities:
Derivatives	(12,892)		(31,020)
Line of Credit Facility, Amount Outstanding	390,000		0
Asset Retirement Obligations, Noncurrent	123,905		115,659
Asset Retirement Obligation, Legally Restricted Assets, Fair Value	452		3,033
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]
Disposal Group, Including Discontinued Operation, Long Lived Assets, Current	0	[1]	0	[1]
Liabilities:
Asset Retirement Obligations, Noncurrent	0
Asset Retirement Obligation, Legally Restricted Assets, Fair Value	0
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]
Disposal Group, Including Discontinued Operation, Long Lived Assets, Current	0	[1]	0	[1]
Liabilities:
Asset Retirement Obligations, Noncurrent	0
Asset Retirement Obligation, Legally Restricted Assets, Fair Value	0
Fair Value, Measurements, Nonrecurring [Member] | Level 3
Assets:
Proved oil and gas properties	9,443	[1]	62,178	[1]
Unproved oil and gas properties	43,252	[1]	3,280	[1]
Disposal Group, Including Discontinued Operation, Long Lived Assets, Current	17,952	[1]	650	[1]
Liabilities:
Asset Retirement Obligations, Noncurrent	923
Asset Retirement Obligation, Legally Restricted Assets, Fair Value	452
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	41,705	[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	63,805	[2]	52,510	[2]
Liabilities:
Derivatives	12,892	[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.