CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 193,468	$ 69,984	$ 27,609
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	19,190	18,918	18,374
Amortization of intangibles	5,213	5,267	6,083
Loss on disposal of property and equipment	249	553	360
Stock-based compensation expense	23,347	21,580	24,677
Amortization of premium (accretion of discount) on marketable securities	(192)	227	1,100
Deferred income taxes	4,019	(4,465)	15,838
Increase (decrease) in accounts receivable allowances	57	(28)	209
Change in operating assets and liabilities:
Accounts receivable	(13,259)	5,754	(10,479)
Inventories	(9,523)	(23,770)	(4,523)
Prepaid expenses and other assets	(2,132)	(1,495)	(17,646)
Accounts payable	(6,556)	1,336	396
Taxes payable and accrued liabilities	10,618	(9,897)	20,041
Net cash provided by operating activities	224,499	83,964	82,039
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(24,114)	(24,677)	(32,496)
Acquisition of technology licenses	(1,026)	(900)	0
Purchases of marketable securities	(207,240)	(62,833)	(151,663)
Proceeds from sales and maturities of marketable securities	70,334	157,551	149,443
Net cash provided by (used in) investing activities	(162,046)	69,141	(34,716)
CASH FLOWS FROM FINANCING ACTIVITIES:
Issuance of common stock under employee stock plans	9,908	9,353	10,020
Repurchase of common stock	(7,302)	(103,153)	(9,188)
Payments of dividends to stockholders	(20,506)	(18,823)	(16,634)
Proceeds from draw on line of credit	0	8,000	5,000
Payments on line of credit	0	(8,000)	(5,000)
Net cash used in financing activities	(17,900)	(112,623)	(15,802)
NET INCREASE IN CASH AND CASH EQUIVALENTS	44,553	40,482	31,521
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	134,137	93,655	62,134
CASH AND CASH EQUIVALENTS AT END OF PERIOD	178,690	134,137	93,655
SUPPLEMENTAL DISCLOSURE OF NON-CASH INVESTING AND FINANCING ACTIVITIES:
Unpaid property and equipment	4,355	1,818	4,913
Unpaid technology licenses	0	100	0
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Cash paid (refund) for income taxes, net of refunds (Note 11)	$ 21,327	$ 7,437	$ (1,571)