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Employee Benefit Plan (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2014
Dec. 31, 2017
Investment            
Assets contributed to 401(k) Plan         $ 7,900  
Restricted investments $ 3,429   $ 3,429     $ 4,469
401(k) Plan distribution period (in years)     3 years      
Compensation expense for assets allocated to participants     $ 1,100 $ 1,200    
Realized gain (loss)   $ (100) (961) 10,757    
Restricted | Maximum            
Investment            
Realized gain (loss) $ (100) $ 100 $ (100) $ 100