Annual report pursuant to Section 13 and 15(d)

Asset Retirement Obligations - Reconciliation of Asset Retirement Obligations Liability (Details)

Asset Retirement Obligations - Reconciliation of Asset Retirement Obligations Liability (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]      
Asset retirement obligations, beginning of period $ 300,446 $ 334,438  
Liabilities settled (28,617) (72,409) $ (72,320)
Accretion of discount 18,431 17,172 17,571
Liabilities incurred and assumed through acquisition 4,286 163  
Revisions of estimated liabilities [1],[2] 15,591 21,082  
Asset retirement obligations, end of period 310,137 300,446 $ 334,438
Less current portion 24,994 23,613  
Long-term $ 285,143 $ 276,833  
[1] Revisions in 2017 were primarily related to increased costs associated with wells at four fields that experienced sustained casing pressure issues. Wells that experience sustained casing pressure require more days and greater work scope to complete the abandonment project. Partially offsetting are downward revisions to cost estimates from service providers for plug and abandonment work at certain locations.
[2] Revisions in 2018 reflect cost estimate increases as a result of new data on the required scope of work becoming available to us through 2018. This new data included data realized during the planning phase of the projects, and as the projects proceeded through the execution phase. This new data indicated that the scope was larger and more difficult than the scope used for end of 2017 estimates. As an example, larger heavy lift vessels would be needed for certain platform removals, and certain wells needed additional well plugging operations to complete the decommissioning per agency requirements.