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Debt - Long-Term Debt (Details) - USD ($)
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
May 23, 2016
May 22, 2016
Aug. 10, 2015
Long-term debt          
Total credit facility $ 300,000,000        
Amount available 248,060,000 $ 221,635,000      
Long-term Line of Credit, Noncurrent 50,000,000 76,300,000      
Letters of Credit Outstanding, Amount 1,940,000 2,065,000      
Line of Credit | Revolving Credit Facility Due 2020          
Long-term debt          
Total credit facility   300,000,000 $ 300,000,000 $ 200,000,000 $ 200,000,000
Long-term Line of Credit, Noncurrent $ 50,000,000 $ 76,300,000      
Weighted-average interest rate 3.10% 2.30%      
Commitment fee percentage per annum 0.20% 0.25%