Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jul. 01, 2018
Cash flows from operating activities:
Net income	$ 81,853	$ 93,413
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense	23,248	22,196
Depreciation of property, plant, and equipment	10,537	8,874
Amortization of intangible assets	1,538	1,538
Amortization of discounts or premiums on investments	(342)	94
Realized (gain) loss on sale of investments	(422)	(269)
Revaluation of contingent consideration	(863)	(254)
Change in deferred income taxes	(1,311)	(2,385)
Accounts receivable	11,383	(18,779)
Unbilled revenue	(1,989)	3,803
Inventories	10,394	(23,220)
Prepaid expenses and other current assets	(463)	(23,112)
Accounts payable	(4,528)	(10,889)
Accrued expenses	(12,270)	(5,666)
Accrued income taxes	(4,000)	(5,360)
Deferred revenue and customer deposits	8,575	32,470
Other	(1,190)	162
Net cash provided by operating activities	120,150	72,616
Cash flows from investing activities:
Purchases of investments	(664,896)	(336,189)
Maturities and sales of investments	587,175	431,008
Purchases of property, plant, and equipment	(8,969)	(21,675)
Net cash provided by (used in) investing activities	(86,690)	73,144
Cash flows from financing activities:
Issuance of common stock under stock plans	26,417	14,811
Repurchase of common stock	(61,690)	(121,308)
Payment of dividends	(17,146)	(15,524)
Payment of contingent consideration	0	(1,000)
Net cash provided by (used in) financing activities	(52,419)	(123,021)
Effect of foreign exchange rate changes on cash and cash equivalents	(149)	(119)
Net change in cash and cash equivalents	(19,108)	22,620
Cash and cash equivalents at beginning of period	108,212	106,582
Cash and cash equivalents at end of period	$ 89,104	$ 129,202