Consolidated Statements of Cash Flows $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Cash flows from operating activities:
Net income	$ 65,203	$ 52,720	$ 35,172
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization of intangible assets	16,101	14,674	13,783
(Gain) loss on sales or write-off of property and equipment	(3)	57	(339)
Amortization of premium on available-for-sale investments	1,976	1,019	463
(Gain) loss on deferred compensation plan investments	(2,531)	(1,257)	133
Change in fair value of contingent consideration			(2,507)
Deferred taxes, net	(15,238)	(5)	42
Excess tax benefits from equity awards		(1,465)	(5,928)
Stock-based compensation expense	52,617	44,989	41,563
Changes in operating assets and liabilities:
Accounts receivable	(3,785)	(3,421)	(5,201)
Inventories	(27,795)	(8,323)	(22,210)
Other assets	1,603	(11,021)	(390)
Accounts payable	3,077	5,483	147
Accrued compensation and related benefits	2,397	3,136	1,068
Accrued liabilities	10,852	8,035	9,942
Income tax liabilities	29,347	3,165	3,998
Net cash provided by operating activities	133,821	107,786	69,736
Cash flows from investing activities:
Property and equipment purchases	(65,773)	(37,112)	(16,024)
Proceeds from sales of property and equipment	3		340
Purchases of short-term investments	(140,531)	(236,912)	(223,018)
Proceeds from maturities and sales of short-term investments	77,502	223,344	189,549
Contributions to deferred compensation plan, net	(5,261)	(5,046)	(8,044)
Net cash used in investing activities	(134,060)	(55,726)	(57,197)
Cash flows from financing activities:
Property and equipment purchased on extended payment terms	(250)	(300)	(300)
Proceeds from exercise of stock options	150	1,344	7,744
Proceeds from shares issued under the employee stock purchase plan	2,701	2,463	2,227
Repurchase of common shares			(32,286)
Dividends and dividend equivalents paid	(33,926)	(33,099)	(29,965)
Excess tax benefits from equity awards		1,465	5,928
Net cash used in financing activities	(31,325)	(28,127)	(46,652)
Effect of change in exchange rates	1,620	(2,090)	(1,293)
Net increase (decrease) in cash and cash equivalents	(29,944)	21,843	(35,406)
Cash and cash equivalents, beginning of period	112,703	90,860	126,266
Cash and cash equivalents, end of period	82,759	112,703	90,860
Supplemental disclosures for cash flow information:
Cash paid for taxes and interest	3,619	1,234	3,322
Supplemental disclosures of non-cash investing and financing activities:
Liability accrued for property and equipment purchases	3,061	787	2,184
Liability accrued for dividends and dividend equivalents	$ 10,686	$ 10,416	$ 10,109