Stockholders' Equity and Stock-Based Compensation - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense related to employee stock options and employee stock purchases	$ 2,679	$ 2,163	$ 5,017	$ 4,327
Cost of products [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense related to employee stock options and employee stock purchases	133	215	231	412
Cost of service [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense related to employee stock options and employee stock purchases	186	157	341	302
Research and development [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense related to employee stock options and employee stock purchases	506	355	983	754
Selling [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense related to employee stock options and employee stock purchases	737	528	1,270	1,067
General and administrative [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense related to employee stock options and employee stock purchases	$ 1,117	$ 908	$ 2,192	$ 1,792