Long-Term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Total debt
Amounts payable within one year	$ 0	$ 0
Long-term debt, noncurrent	25,000	35,287
Revolving credit facility [Member]
Total debt
Long-term debt, noncurrent	$ 25,000	$ 35,287