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Fair Value Measurements (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Proved Oil and Gas Properties, Other Property, Plant and Equipment and Other Assets [Abstract]    
Proved Oil and Gas Properties and Other Assets Carrying Amount $ 208,000  
Proved Oil and Gas Properties and Other Assets Written Down During the Period - Fair value at End of Period 183,000  
Pretax Impairment Charges for Proved Oil and Gas Properties and Other Assets 25,000  
Pretax Impairment Charge for a Commodity Price-Related Write-Down of Other Assets 24,000  
Debt Disclosure [Abstract]    
Aggregate Principal Amount of Current and Long-Term Debt 6,040,000 $ 6,040,000
Estimated Fair Value of Debt 6,168,000 6,027,000
Commodity Contract [Member] | Crude Oil | Price Swaps    
Financial Liabilities:    
Financial and Nonfinancial Liabilities, Fair Value Disclosure 2,124  
Commodity Contract [Member] | Crude Oil | Price Swaps | Fair Value, Inputs, Level 1 [Member]    
Financial Liabilities:    
Financial and Nonfinancial Liabilities, Fair Value Disclosure 0  
Commodity Contract [Member] | Crude Oil | Price Swaps | Fair Value, Inputs, Level 2 [Member]    
Financial Liabilities:    
Financial and Nonfinancial Liabilities, Fair Value Disclosure 2,124  
Commodity Contract [Member] | Crude Oil | Price Swaps | Fair Value, Inputs, Level 3 [Member]    
Financial Liabilities:    
Financial and Nonfinancial Liabilities, Fair Value Disclosure 0  
Commodity Contract [Member] | Crude Oil | Basis Swaps    
Financial Liabilities:    
Financial and Nonfinancial Liabilities, Fair Value Disclosure 2,687 23,806
Commodity Contract [Member] | Crude Oil | Basis Swaps | Fair Value, Inputs, Level 1 [Member]    
Financial Liabilities:    
Financial and Nonfinancial Liabilities, Fair Value Disclosure 0 0
Commodity Contract [Member] | Crude Oil | Basis Swaps | Fair Value, Inputs, Level 2 [Member]    
Financial Liabilities:    
Financial and Nonfinancial Liabilities, Fair Value Disclosure 2,687 23,806
Commodity Contract [Member] | Crude Oil | Basis Swaps | Fair Value, Inputs, Level 3 [Member]    
Financial Liabilities:    
Financial and Nonfinancial Liabilities, Fair Value Disclosure 0 $ 0
Commodity Contract [Member] | Natural Gas | Price Swaps    
Financial Assets: [Abstract]    
Assets, Fair Value Disclosure 7,974  
Commodity Contract [Member] | Natural Gas | Price Swaps | Fair Value, Inputs, Level 1 [Member]    
Financial Assets: [Abstract]    
Assets, Fair Value Disclosure 0  
Commodity Contract [Member] | Natural Gas | Price Swaps | Fair Value, Inputs, Level 2 [Member]    
Financial Assets: [Abstract]    
Assets, Fair Value Disclosure 7,974  
Commodity Contract [Member] | Natural Gas | Price Swaps | Fair Value, Inputs, Level 3 [Member]    
Financial Assets: [Abstract]    
Assets, Fair Value Disclosure $ 0