Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income (loss)	$ 29,993	$ 2,577	$ (4,440)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Stock-based compensation	5,110	3,713	4,084
Depreciation and amortization	1,016	982	1,186
Change in deferred tax assets	(18,678)	22	(55)
Other	(52)	(7)	282
Changes in assets and liabilities:
Accounts receivable	(4,229)	(4,899)	(536)
Inventories	(13,805)	(2,899)	(1,090)
Other current assets and prepaid expenses	(591)	(432)	241
Other long-term assets	6	4	(23)
Accounts payable	4,404	639	(1,124)
Accrued liabilities	9,345	3,461	2,687
Other long-term liabilities		(329)	(289)
Deferred revenue	1,557	(826)	(2,319)
Income tax liability	211	(14)	37
Net cash provided by (used in) operating activities	14,287	1,992	(1,359)
Cash flows from investing activities:
Acquisition of property, equipment and software	(855)	(537)	(746)
Disposal of property and equipment	53	20
Proceeds from sales of marketable investments	33,640	9,008	21,171
Proceeds from maturities of marketable investments	45,812	25,810	35,918
Purchase of marketable investments	(60,956)	(37,693)	(23,697)
Net cash provided by (used in) investing activities	17,694	(3,392)	32,646
Cash flows from financing activities:
Repurchase of common stock	(35,167)	(4,873)	(40,085)
Proceeds from exercise of stock options and employee stock purchase plan	5,435	10,111	11,079
Taxes paid related to net share settlement of equity awards	(1,469)	(618)	(1,018)
Payments on capital lease obligation	(371)	(313)	(198)
Net cash provided by (used in) financing activities	(31,572)	4,307	(30,222)
Net increase in cash and cash equivalents	409	2,907	1,065
Cash and cash equivalents at beginning of year	13,775	10,868	9,803
Cash and cash equivalents at end of year	14,184	13,775	10,868
Supplemental cash flow information:
Cash paid for interest	70	43	20
Cash paid for income taxes	220	222	160
Supplemental non-cash investing and financing activities:
Assets acquired under capital lease	$ 365	$ 801	$ 285