Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Oct. 01, 2017
Cash flows from operating activities:
Net income	$ 173,849	$ 204,459
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense	31,339	23,355
Depreciation of property, plant, and equipment	13,474	9,811
Amortization of intangible assets	2,307	2,539
Amortization of discounts or premiums on investments	123	188
Realized (gain) loss on sale of investments	(535)	(370)
Revaluation of contingent consideration	(50)	88
Change in deferred income taxes	175	(1,830)
Accounts receivable	(18,136)	(58,379)
Unbilled revenue	(6,542)	(46,010)
Inventories	(27,699)	(18,550)
Prepaid expenses and other current assets	(3,437)	(14,666)
Accounts payable	(1,182)	19,132
Accrued expenses	2,266	8,819
Accrued income taxes	(17,497)	3,724
Deferred revenue and customer deposits	4,841	5,883
Other	(1,604)	1,861
Net cash provided by operating activities	151,692	140,054
Cash flows from investing activities:
Purchases of investments	(616,047)	(415,508)
Maturities and sales of investments	635,119	428,076
Purchases of property, plant, and equipment	(27,356)	(20,044)
Cash paid for acquisition of business	0	(24,118)
Net cash provided by (used in) investing activities	0	(291)
Net cash provided by (used in) investing activities	(8,284)	(31,885)
Cash flows from financing activities:
Issuance of common stock under stock plans	25,882	44,750
Repurchase of common stock	(142,262)	(99,347)
Payment of dividends	(23,283)	(21,236)
Payment of contingent consideration	(1,000)	(1,926)
Net cash provided by (used in) financing activities	(140,663)	(77,759)
Effect of foreign exchange rate changes on cash and cash equivalents	(1,956)	1,298
Net change in cash and cash equivalents	789	31,708
Cash and cash equivalents at beginning of period	106,582	79,641
Cash and cash equivalents at end of period	$ 107,371	$ 111,349