Fair Value Measurements (Details) - USD ($) $ in Thousands		Dec. 31, 2015		Dec. 31, 2014
Assets:
Derivatives		$ 488,411		$ 592,208
Proved oil and gas properties		7,606,405		7,348,436
Property, Plant and Equipment, Other, Net		153,100		334,356
Oil and gas properties held for sale		641		17,891
Liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability		8		70
Net Profits Plan Liability Noncurrent		7,611		27,136
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Derivatives		0	[1]	0	[2]
Liabilities [Abstract]
Net Profits Plan Liability Noncurrent		0	[1]	0	[2]
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Not Designated as Hedging Instrument [Member]
Liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability		0	[1]	0	[2]
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets:
Derivatives		488,411	[2]	592,208	[1]
Liabilities [Abstract]
Net Profits Plan Liability Noncurrent		0	[1]	0	[2]
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Not Designated as Hedging Instrument [Member]
Assets:
Derivatives		488,411		592,208
Liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability		8	[2]	70	[1]
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets:
Derivatives		0	[1]	0	[2]
Liabilities [Abstract]
Net Profits Plan Liability Noncurrent		7,611	[1]	27,136	[2]
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member]
Liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability		0	[1]	0	[2]
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Proved oil and gas properties		0	[3]	0	[4]
Property, Plant and Equipment, Other, Net	[4]	0
Oil and gas properties held for sale	[4]			0
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets:
Proved oil and gas properties		0	[3]	0	[4]
Property, Plant and Equipment, Other, Net	[4]	0
Oil and gas properties held for sale	[4]			0
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets:
Proved oil and gas properties		124,184	[3]	33,423	[4]
Property, Plant and Equipment, Other, Net	[4]	$ 629
Oil and gas properties held for sale	[4]			$ 17,891

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

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

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

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