Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 29, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net income	$ 123,538	$ 173,849
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense	33,442	31,339
Depreciation of property, plant, and equipment	15,800	13,474
Amortization of intangible assets	2,119	2,307
Amortization of discounts or premiums on investments	(563)	123
Realized (gain) loss on sale of investments	(716)	(535)
Revaluation of contingent consideration	(1,401)	(50)
Change in deferred income taxes	(1,805)	175
Accounts receivable	11,387	(18,136)
Unbilled revenue	(2,900)	(6,542)
Inventories	18,019	(27,699)
Prepaid expenses and other current assets	5,827	(3,437)
Accounts payable	(3,398)	(1,182)
Accrued expenses	(11,827)	2,266
Accrued income taxes	(1,043)	(17,497)
Deferred revenue and customer deposits	698	4,841
Other	(1,863)	(1,604)
Net cash provided by operating activities	185,314	151,692
Cash flows from investing activities:
Purchases of investments	(883,363)	(616,047)
Maturities and sales of investments	781,929	635,119
Purchases of property, plant, and equipment	(13,518)	(27,356)
Net cash provided by (used in) investing activities	(114,952)	(8,284)
Cash flows from financing activities:
Issuance of common stock under stock plans	31,780	25,882
Repurchase of common stock	(61,690)	(142,262)
Payment of dividends	(25,682)	(23,283)
Payment of contingent consideration	0	(1,000)
Net cash provided by (used in) financing activities	(55,592)	(140,663)
Effect of foreign exchange rate changes on cash and cash equivalents	(1,281)	(1,956)
Net change in cash and cash equivalents	13,489	789
Cash and cash equivalents at beginning of period	108,212	106,582
Cash and cash equivalents at end of period	$ 121,701	$ 107,371