Summary of Significant Accounting Policies - Reconciliation of Restructuring Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Reserve [Roll Forward]
Charged to restructuring costs	$ 1,026	$ 753	$ 1,274
Cost Optimization Project | On-Going Benefit Costs
Restructuring Reserve [Roll Forward]
Balance at beginning of period	325	44	0
Charged to restructuring costs	522	563	633
Payments	(760)	(282)	(589)
Balance at end of period	87	325	44
Cost Optimization Project | Exit Costs
Restructuring Reserve [Roll Forward]
Balance at beginning of period	273	338	0
Charged to restructuring costs	504	190	641
Payments	(336)	(255)	(303)
Balance at end of period	$ 441	$ 273	$ 338