Fair Value Measurements Fair Value (Details) - USD ($)	Jun. 30, 2015		Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 400,486,000		$ 592,208,000
Proved Oil and Gas Property, Successful Effort Method	7,356,877,000		7,348,436,000
Disposal Group, Including Discontinued Operation, Assets, Current	7,361,000		17,891,000
Derivative Liability, Fair Value, Gross Liability	9,133,000		70,000
Net Profits Plan liability	18,326,000		27,136,000
Asset Retirement Obligations Assets Held-for-sale Noncurrent	200,000
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	18,326,000	[1]	27,136,000	[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	7,700,000	[3]	33,400,000	[4]
Disposal Group, Including Discontinued Operation, Assets, Current	5,801,000	[3]	17,891,000	[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	400,486,000	[1]	592,208,000	[2]
Derivative Liability, Fair Value, Gross Liability	9,133,000	[1]	70,000	[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 or liability 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.