Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Apr. 01, 2018
Cash flows from operating activities:
Net income	$ 33,104	$ 37,217
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense	12,281	13,194
Depreciation of property, plant, and equipment	5,175	4,430
Amortization of intangible assets	769	769
Amortization of discounts or premiums on investments	(1,461)	56
Realized (gain) loss on sale of investments	(40)	(22)
Revaluation of contingent consideration	(985)	(442)
Change in deferred income taxes	(1,140)	(1,469)
Change in operating assets and liabilities:
Accounts receivable	12,827	24,830
Unbilled revenue	(1,890)	3,389
Inventories	4,073	(27,321)
Prepaid expenses and other current assets	3,191	(866)
Operating Lease, Payments, Use		0
Accounts payable	4,245	5,309
Accrued expenses	(11,708)	(9,927)
Accrued income taxes	(3,501)	186
Deferred revenue and customer deposits	6,760	5,877
Other	(439)	(424)
Net cash provided by operating activities	61,261	54,786
Cash flows from investing activities:
Purchases of investments	(258,596)	(118,108)
Maturities and sales of investments	193,572	144,337
Purchases of property, plant, and equipment	(5,078)	(13,270)
Net cash provided by (used in) investing activities	(70,102)	12,959
Cash flows from financing activities:
Issuance of common stock under stock plans	14,346	11,123
Repurchase of common stock	0	(69,202)
Payment of dividends	(8,564)	(7,787)
Net cash provided by (used in) financing activities	5,782	(65,866)
Effect of foreign exchange rate changes on cash and cash equivalents	143	1,111
Net change in cash and cash equivalents	(2,916)	2,990
Cash and cash equivalents at beginning of period	108,212	106,582
Cash and cash equivalents at end of period	$ 105,296	$ 109,572