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Debt - Long-Term Debt (Details) - USD ($)
9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2016
May 23, 2016
May 22, 2016
Aug. 10, 2015
Long-term debt          
Revolving credit facility $ 82,300,000 $ 89,100,000      
Line of Credit | Revolving Credit Facility Due 2020          
Long-term debt          
Revolving credit facility $ 82,300,000 $ 89,100,000      
Weighted-average interest rate 2.30% 1.90%      
Amount available $ 215,635,000 $ 208,735,000      
Total credit facility 300,000,000 300,000,000 $ 300,000,000 $ 200,000,000 $ 200,000,000
Standby letters of credit $ 2,065,000 $ 2,165,000      
Commitment fee percentage per annum 0.25% 0.25%