Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Apr. 01, 2017
Cash flows from operating activities:
Net income	$ 16,381	$ 5,352
Reconciliation of net income to net cash provided by operating activities:
Depreciation and amortization	1,724	1,854
Stock-based compensation	2,338	2,164
Disposal of fixed assets	45	63
Inventory write-down	95	406
Deferred income taxes	2,062	(479)
Changes in assets and liabilities:
Accounts receivable	8,035	(6,874)
Inventories	315	(2,695)
Inventories-delivered systems	41	(823)
Prepaid expenses and other	(454)	(88)
Accounts payable, accrued and other liabilities	(1,474)	970
Deferred revenue	2,172	3,039
Income taxes payable	1,434	186
Net cash provided by operating activities	32,714	3,075
Cash flows from investing activities:
Payments to acquire certain assets	(1,000)
Sales of marketable securities	17,435	10,181
Maturities of marketable securities	6,500	24,531
Purchases of marketable securities		(36,514)
Purchases of property, plant and equipment	(1,319)	(47)
Net cash provided by (used in) investing activities	21,616	(1,849)
Cash flows from financing activities:
Proceeds from sale of shares under employee stock option plans and purchase plan	545	1,217
Taxes paid on net issuance of stock awards	(476)	(1,755)
Net cash used in financing activities	(22,918)	(538)
Effect of exchange rate changes on cash and cash equivalents	(399)	(333)
Net increase in cash and cash equivalents	31,013	355
Cash and cash equivalents, beginning of period	34,899	47,062
Cash and cash equivalents, end of period	65,912	47,417
Supplemental disclosure of non-cash investing activities:
Transfer of property, plant and equipment to inventory, net	(91)	22
Property, plant and equipment included in accounts payable	265	$ 789
Share Repurchase Plans [Member]
Cash flows from financing activities:
Repurchases of common stock	$ (22,987)