Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued expenses
Salaries and benefits	$ 7,875	$ 11,662
Interest	2,890	2,583
Other taxes	971	1,048
Accrued expenses	11,736	15,293
Other current liabilities
Other postretirement benefits, current	3,226	3,332
Installment financing contracts, current	2,978	2,157
Other	570	838
Other current liabilities	6,774	6,327
Other liabilities, noncurrent
Deferred revenue, noncurrent	28,036	7,453
Installment financing contracts, noncurrent	8,982	8,692
Other	1,909	2,094
Other liabilities, noncurrent	$ 38,927	$ 18,239