Severance and Other Charges - Accrual for Severance and Other Exit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accrual for severance and other exit costs
Balance beginning of period	$ 1,738	$ 1,092
Provision	(480)	2,555
Payments	(1,230)	(1,909)
Balance end of period	28	1,738
Employee severance expense
Accrual for severance and other exit costs
Balance beginning of period	718	1,092
Provision	139	1,227
Payments	(829)	(1,601)
Balance end of period	28	718
Lease exit costs
Accrual for severance and other exit costs
Balance beginning of period	1,020	0
Provision	(619)	1,328
Payments	(401)	(308)
Balance end of period	$ 0	$ 1,020