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Fair Value Measurements Proved and Unproved Oil and Gas Properties (Details)
 3 Months Ended 9 Months Ended
Sep. 30, 2015
USD ($)
A
Sep. 30, 2014
USD ($)
Sep. 30, 2015
USD ($)
A
Sep. 30, 2014
USD ($)
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%    
Impairment of Oil and Gas Properties $ 55,990,000 $ 0 $ 124,430,000 $ 0  
Proved Oil and Gas Property, Successful Effort Method 7,468,331,000   7,468,331,000   $ 7,348,436,000
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down     98,600,000    
Abandonment and impairment of unproved properties 6,600,000 $ 15,522,000 24,046,000 $ 18,487,000  
Unproved Oil and Gas Property, Successful Effort Method 381,869,000   381,869,000   532,498,000
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 56,849,000 [1]   56,849,000 [1]   33,423,000 [2]
Unproved Oil and Gas Property, Successful Effort Method $ 0   $ 0   $ 0
Oil and Gas Properties [Member]          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Number of Periods Used for Price Assumptions of Strip Prices | A 5   5    
[1] This represents a non-financial asset 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.