Fair Value Measurements Proved and Unproved Oil and Gas Properties (Details) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2015 USD ($) A		Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) A		Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount Rate Used for Fair Value of Oil and Gas Properties	12.00%			12.00%
Number of Periods Used for Price Assumptions of Strip Prices of Liabilities | A	5			5
Impairment of Oil and Gas Properties	$ 12,914		$ 0	$ 68,440		$ 0
Proved Oil and Gas Property, Successful Effort Method	7,356,877			7,356,877			$ 7,348,436
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down	66,000			99,900
Abandonment and impairment of unproved properties	5,819		$ 164	17,446		$ 2,965
Unproved Oil and Gas Property, Successful Effort Method	419,903			419,903			532,498
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	[1]		7,700	[1]		$ 33,400	[2]
Unproved Oil and Gas Property, Successful Effort Method	$ 0			$ 0

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

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