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Income Taxes (Details) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Income Taxes          
Effective tax rate     21.00%   35.00%
Income tax (benefit) expense $ 1,781,000 $ 942,000 $ 5,427,000 $ 3,884,000  
Fuel tax credits 1,100,000 $ 900,000 $ 2,100,000 $ 1,600,000  
Withholding tax on dividends     10.00%    
Dividend withholding tax between countries     5.00%    
Liability for uncertain tax positions $ 0   $ 0