Stock-Based Compensation - Total Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Pre-tax stock-based compensation expense	$ 7,166	$ 5,366	$ 5,380
Less: income tax effect	(2,795)	(2,093)	(2,098)
Net (after tax) stock-based compensation expense	4,371	3,273	3,282
Costs of sales
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Pre-tax stock-based compensation expense	1,750	1,396	1,447
Operating expenses
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Pre-tax stock-based compensation expense	$ 5,416	$ 3,970	$ 3,933