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Claim Settlement Receivable - (Details) - USD ($)
$ in Thousands
1 Months Ended 3 Months Ended 6 Months Ended 9 Months Ended
Mar. 24, 2016
Oct. 31, 2017
Oct. 31, 2016
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Receivables with Imputed Interest [Line Items]                  
Litigation settlement receivable       $ 7,982     $ 7,982   $ 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 $ 200 $ 200    
Other income | BP Exploration & Production Inc.                  
Receivables with Imputed Interest [Line Items]                  
Litigation settlement gain (loss)               $ 12,500  
Subsequent Event | BP Exploration & Production Inc.                  
Receivables with Imputed Interest [Line Items]                  
Litigation settlement amount received   $ 2,700