Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 27, 2020	Sep. 29, 2019
Cash flows from operating activities:
Net income	$ 106,841	$ 123,538
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense	32,076	33,442
Depreciation of property, plant, and equipment	16,467	15,800
Amortization of intangible assets	3,437	2,119
Intangible asset impairment charges	19,571	0
Excess and obsolete inventory charges	9,386	4,121
Operating lease asset impairment charges	2,534	0
Amortization of discounts or premiums on investments	765	(563)
Realized gain on sale of investments	(3,591)	(716)
Credit loss on investments	75	0
Revaluation of contingent consideration	(114)	(1,401)
Change in deferred income taxes	3,194	(1,805)
Accounts receivable	(23,878)	11,387
Unbilled revenue	(10,606)	(2,900)
Inventories	(3,065)	13,898
Prepaid expenses and other current assets	(10,718)	5,827
Accounts payable	3,587	(3,398)
Accrued expenses	17,028	(11,827)
Accrued income taxes	(23,627)	(1,043)
Deferred revenue and customer deposits	19,066	698
Other	1,022	(1,616)
Net cash provided by operating activities	159,450	185,561
Cash flows from investing activities:
Purchases of investments	(601,447)	(883,363)
Maturities and sales of investments	567,245	781,929
Purchases of property, plant, and equipment	(9,829)	(13,518)
Net cash provided by (used in) investing activities	(44,031)	(114,952)
Cash flows from financing activities:
Issuance of common stock under stock plans	91,390	31,780
Repurchase of common stock	(51,036)	(61,690)
Payment of dividends	(28,554)	(25,682)
Payment of contingent consideration	(1,039)	0
Net cash provided by (used in) financing activities	10,761	(55,592)
Effect of foreign exchange rate changes on cash and cash equivalents	745	(1,528)
Net change in cash and cash equivalents	126,925	13,489
Cash and cash equivalents at beginning of period	171,431	108,212
Cash and cash equivalents at end of period	$ 298,356	$ 121,701