Quarterly report pursuant to Section 13 or 15(d)

Property and Equipment (Summary of Non-producing Properties Excluded from Amortization by Area) (Details)

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Property and Equipment (Summary of Non-producing Properties Excluded from Amortization by Area) (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]    
Total non-producing properties not subject to amortization $ 2,971,119 $ 2,912,974
Utica    
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]    
Total non-producing properties not subject to amortization 1,548,121  
MidContinent    
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]    
Total non-producing properties not subject to amortization 1,421,853  
Niobrara    
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]    
Total non-producing properties not subject to amortization 449  
Southern Louisiana    
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]    
Total non-producing properties not subject to amortization 552  
Bakken    
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]    
Total non-producing properties not subject to amortization 99  
Other    
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]    
Total non-producing properties not subject to amortization $ 45