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Debt - Narratives (Details) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Aug. 10, 2015
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
May 23, 2016
May 22, 2016
Line of Credit Facility                
Line of credit maximum borrowing amount   $ 300,000,000   $ 300,000,000        
Debt amortization expense   46,000 $ 46,000 93,000 $ 0      
Long-term Line of Credit, Noncurrent   57,000,000   $ 57,000,000   $ 76,300,000    
Line of Credit | Revolving Credit Facility Due 2020                
Line of Credit Facility                
Debt instrument, term 5 years              
Line of credit maximum borrowing amount $ 200,000,000         $ 300,000,000 $ 300,000,000 $ 200,000,000
Line of credit facility contingent increase to maximum borrowing capacity             $ 100,000,000  
Commitment fee percentage per annum       0.25%   0.25%    
Long-term Line of Credit, Noncurrent   $ 57,000,000   $ 57,000,000   $ 76,300,000    
Line of Credit | Revolving Credit Facility Due 2020 Swingline Sublimit                
Line of Credit Facility                
Line of credit maximum borrowing amount 15,000,000              
Line of Credit | Revolving Credit Facility Due 2020 Letter Of Credit Sublimit                
Line of Credit Facility                
Line of credit maximum borrowing amount $ 10,000,000              
Minimum | Line of Credit | Revolving Credit Facility Due 2020                
Line of Credit Facility                
Commitment fee percentage per annum 0.20%              
Maximum | Line of Credit | Revolving Credit Facility Due 2020                
Line of Credit Facility                
Commitment fee percentage per annum 0.40%