Fair Value Measurements Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2015		Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 494,829		$ 592,208
Proved Oil and Gas Property, Successful Effort Method	7,468,331		7,348,436
Disposal Group, Including Discontinued Operation, Assets, Current	29,173		17,891
Derivative Liability, Fair Value, Gross Liability	4,919		70
Net Profits Plan liability	13,962		27,136
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net Profits Plan liability	0	[1]	0	[2]
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net Profits Plan liability	0	[1]	0	[2]
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net Profits Plan liability	13,962	[1]	27,136	[2]
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Proved Oil and Gas Property, Successful Effort Method	0	[3]	0	[4]
Disposal Group, Including Discontinued Operation, Assets, Current	0	[3]	0	[4]
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Proved Oil and Gas Property, Successful Effort Method	0	[3]	0	[4]
Disposal Group, Including Discontinued Operation, Assets, Current	0	[3]	0	[4]
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Proved Oil and Gas Property, Successful Effort Method	56,849	[3]	33,423	[4]
Disposal Group, Including Discontinued Operation, Assets, Current	3,376	[3]	17,891	[4]
Derivatives not designated as hedging instruments | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	[1]	0	[2]
Derivative Liability, Fair Value, Gross Liability	0	[1]	0	[2]
Derivatives not designated as hedging instruments | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	494,829	[1]	592,208	[2]
Derivative Liability, Fair Value, Gross Liability	4,919	[1]	70	[2]
Derivatives not designated as hedging instruments | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	[1]	0	[2]
Derivative Liability, Fair Value, Gross Liability	$ 0	[1]	$ 0	[2]

[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.