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Debt - Long-Term Debt (Details) - USD ($)
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
May 23, 2016
May 22, 2016
Aug. 10, 2015
Long-term debt            
Total credit facility $ 300,000,000          
Amount available 223,935,000   $ 221,635,000      
Long-term Line of Credit, Noncurrent 74,000,000   76,300,000      
Letters of Credit Outstanding, Amount 2,065,000   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 $ 74,000,000   $ 76,300,000      
Weighted-average interest rate 2.90%   2.30%      
Commitment fee percentage per annum 0.25% 0.25%