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Claim Settlement Receivable (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 9 Months Ended
Oct. 03, 2016
Mar. 24, 2016
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Receivables with Imputed Interest [Line Items]                
Litigation settlement receivable     $ 12,746   $ 12,746 $ 12,746   $ 0
Litigation settlement proceeds     0 $ 0   12,548 $ 0  
BP Exploration & Production Inc.                
Receivables with Imputed Interest [Line Items]                
Litigation settlement receivable, gross   $ 13,200            
Litigation settlement receivable due in October 2017, 2018 and 2019   $ 2,700            
Imputed interest rate on litigation settlement receivable   3.00%            
Litigation settlement receivable   $ 12,500            
Unamortized discount   $ 700            
Interest income     $ 100   $ 200      
Other income | BP Exploration & Production Inc.                
Receivables with Imputed Interest [Line Items]                
Litigation settlement proceeds           $ 12,500    
Subsequent Event | BP Exploration & Production Inc.                
Receivables with Imputed Interest [Line Items]                
Litigation settlement amount received $ 5,000