Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accrued expenses
Salaries and benefits	$ 7,893	$ 7,875
Interest	2,969	2,890
Other taxes	970	971
Accrued expenses	11,832	11,736
Other current liabilities
Other postretirement benefits, current	3,226	3,226
Installment financing contracts, current	194	2,978
Other	514	570
Other current liabilities	3,934	6,774
Other liabilities, noncurrent
Deferred revenue, noncurrent	28,554	28,036
Installment financing contracts, noncurrent	3,233	8,982
Other	1,851	1,909
Other liabilities, noncurrent	$ 33,638	$ 38,927