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Debt - Narratives (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Feb. 12, 2019
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
May 23, 2016
Line of Credit Facility              
Line of credit maximum borrowing amount   $ 300,000   $ 300,000      
Debt amortization expense   42 $ 46 121 $ 139    
Long-term Line of Credit, Noncurrent   112,700   $ 112,700   $ 50,000  
Line of Credit | Revolving Credit Facility Due 2024              
Line of Credit Facility              
Debt instrument, term 5 years            
Line of credit maximum borrowing amount $ 300,000         $ 300,000 $ 300,000
Line of credit facility contingent increase to maximum borrowing capacity 150,000            
Commitment fee percentage per annum       0.20%   0.20%  
Long-term Line of Credit, Noncurrent   $ 112,700   $ 112,700      
Line of Credit | Revolving Credit Facility Due 2024 Swingline Sublimit              
Line of Credit Facility              
Line of credit maximum borrowing amount 15,000            
Line of Credit | Revolving Credit Facility Due 2024 Letter Of Credit Sublimit              
Line of Credit Facility              
Line of credit maximum borrowing amount $ 10,000            
Minimum | Line of Credit | Revolving Credit Facility Due 2024              
Line of Credit Facility              
Commitment fee percentage per annum 0.20%            
Maximum | Line of Credit | Revolving Credit Facility Due 2024              
Line of Credit Facility              
Commitment fee percentage per annum 0.30%