Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Accrued expenses
Salaries and benefits	$ 9,584	$ 11,662
Interest	2,863	2,583
Other taxes	879	1,048
Accrued expenses	13,326	15,293
Other current liabilities
Other postretirement benefits, current	3,465	3,332
Installment financing contracts, current	2,317	2,157
Other	625	838
Other current liabilities	6,407	6,327
Other liabilities, noncurrent
Deferred revenue on undersea fiber circuits, noncurrent	27,978	7,453
Installment financing contracts, noncurrent	9,248	8,692
Other	1,813	2,094
Other liabilities, noncurrent	$ 39,039	$ 18,239