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Claim Settlement Receivable (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Oct. 03, 2016
Mar. 24, 2016
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Receivables with Imputed Interest [Line Items]                
Litigation settlement receivable     $ 7,922     $ 7,922   $ 7,804
Litigation settlement gain (loss)     0   $ 0 0 $ 12,548  
BP Exploration & Production Inc.                
Receivables with Imputed Interest [Line Items]                
Litigation settlement receivable, gross   $ 13,200            
Litigation settlement receivable   12,500            
Litigation settlement amount received $ 5,000              
Litigation settlement receivable due in October 2017, 2018 and 2019   $ 2,700            
Imputed interest rate on litigation settlement receivable   3.00%            
Unamortized discount   $ 700            
Interest income     $ 100 $ 100 $ 100 $ 100    
Other income | BP Exploration & Production Inc.                
Receivables with Imputed Interest [Line Items]                
Litigation settlement gain (loss)             $ 12,500