Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017
Cash flows from operating activities:
Net income	$ 34,056	$ 13,640
Reconciliation of net income to net cash provided by operating activities:
Depreciation and amortization	3,326	3,552
Stock-based compensation	5,017	4,327
Disposal of fixed assets	51	98
Inventory write-down	269	1,015
Deferred income taxes	4,320	798
Changes in assets and liabilities:
Accounts receivable	7,193	(4,720)
Inventories	(1,722)	(8,726)
Inventories-delivered systems	(322)	783
Prepaid expenses and other	(3,422)	(947)
Accounts payable, accrued and other liabilities	7,204	(1,885)
Deferred revenue	2,464	1,837
Income taxes payable	537	486
Net cash provided by operating activities	58,971	10,258
Cash flows from investing activities:
Payments to acquire certain assets	(2,000)	(2,000)
Sales of marketable securities	18,507	27,391
Maturities of marketable securities	17,345	40,923
Purchases of marketable securities	(16,320)	(78,787)
Purchases of property, plant and equipment	(1,761)	(1,536)
Net cash provided by (used in) investing activities	15,771	(14,009)
Cash flows from financing activities:
Proceeds from sale of shares under employee stock option plans and purchase plan	2,709	2,865
Taxes paid on net issuance of stock awards	(2,731)	(3,392)
Net cash used in financing activities	(23,009)	(527)
Effect of exchange rate changes on cash and cash equivalents	(431)	(430)
Net increase (decrease) in cash and cash equivalents	51,302	(4,708)
Cash and cash equivalents, beginning of period	34,899	47,062
Cash and cash equivalents, end of period	86,201	42,354
Supplemental disclosure of non-cash investing activities:
Transfer of property, plant and equipment to inventory	218	1,531
Transfer of inventory to property, plant and equipment	291	980
Property, plant and equipment included in accounts payable, accrued and other liabilities	555	$ 355
Share Repurchase Plans [Member]
Cash flows from financing activities:
Repurchases of common stock	$ (22,987)