Quarterly report pursuant to Section 13 or 15(d)

Asset Retirement Obligations - Summary of Changes to Asset Retirement Obligation (Details)

v3.8.0.1
Asset Retirement Obligations - Summary of Changes to Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]      
Asset retirement obligations, beginning of period $ 334,438    
Asset retirement obligation settlements (56,226) $ (56,167)  
Accretion of discount 12,820    
Revisions of estimated liabilities [1] 13,984    
Asset retirement obligations, end of period 305,016    
Less current portion 29,456   $ 78,264
Long-term $ 275,560   $ 256,174
[1] Revisions were primarily related to changes from sustained casing pressure at four fields. Wells that experience sustained casing pressure require more days and greater work scope to complete the abandonment project.