Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jul. 01, 2018	Jul. 02, 2017
Cash flows from operating activities:
Net income	$ 93,413	$ 101,966
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense	22,196	15,329
Depreciation of property, plant, and equipment	8,874	6,288
Amortization of intangible assets	1,538	1,740
Amortization of discounts or premiums on investments	94	171
Realized (gain) loss on sale of investments	(269)	(107)
Revaluation of contingent consideration	(254)	(151)
Change in deferred income taxes	(2,385)	(972)
Accounts receivable	(18,779)	(18,841)
Unbilled revenue	3,803	(3,315)
Inventories	(23,220)	(7,711)
Prepaid expenses and other current assets	(23,112)	(23,990)
Accounts payable	(10,889)	11,814
Accrued expenses	(5,666)	(157)
Accrued income taxes	(5,360)	(532)
Deferred revenue and customer deposits	32,470	9,080
Other	162	8
Net cash provided by operating activities	72,616	90,620
Cash flows from investing activities:
Purchases of investments	(336,189)	(304,611)
Maturities and sales of investments	431,008	279,654
Purchases of property, plant, and equipment	(21,675)	(12,172)
Cash paid for acquisition of businesses, net of cash acquired	0	(25,519)
Net cash provided by (used in) investing activities	0	(291)
Net cash provided by (used in) investing activities	73,144	(62,939)
Cash flows from financing activities:
Issuance of common stock under stock plans	14,811	35,050
Repurchase of common stock	(121,308)	(62,343)
Payment of dividends	(15,524)	(13,864)
Payment of contingent consideration	(1,000)	0
Net cash provided by (used in) financing activities	(123,021)	(41,157)
Effect of foreign exchange rate changes on cash and cash equivalents	(119)	2,101
Net change in cash and cash equivalents	22,620	(11,375)
Cash and cash equivalents at beginning of period	106,582	79,641
Cash and cash equivalents at end of period	$ 129,202	$ 68,266