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 (loss)	$ (12,348)	$ (30,770)	$ 29,993
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Stock-based compensation	9,832	7,157	5,110
Depreciation and amortization	1,548	1,209	1,016
Amortization of contract acquisition costs	2,915	1,834
Change in deferred tax assets	34	17,438	(18,678)
Provision for doubtful accounts receivable	(590)	(1,257)	1
Other	127	241	(51)
Changes in assets and liabilities:
Accounts receivable	(2,509)	(117)	(4,229)
Inventories	(5,907)	768	(13,805)
Other current assets and prepaid expenses	(1,762)	(1,070)	(591)
Other long-term assets	(3,355)	(2,754)	6
Accounts payable	1,406	4,277	4,404
Accrued liabilities	7,157	(3,781)	9,345
Extended warranty liabilities	(1,160)	3,159
Other long-term liabilities	(140)	140
Deferred revenue	1,656	1,305	1,557
Income tax liability	(301)	15	211
Net cash provided by (used in) operating activities	(2,217)	308	14,287
Cash flows from investing activities:
Acquisition of property, equipment and software	(991)	(1,488)	(855)
Disposal of property and equipment	45	41	53
Proceeds from sales of marketable investments		13,044	33,640
Proceeds from maturities of marketable investments	14,700	10,050	45,812
Purchase of marketable investments	(12,687)	(10,874)	(60,956)
Net cash provided by investing activities	1,067	10,773	17,694
Cash flows from financing activities:
Repurchase of common stock			(35,167)
Proceeds from exercise of stock options and employee stock purchase plan	2,894	4,399	5,435
Taxes paid related to net share settlement of equity awards	(831)	(3,129)	(1,469)
Payments on capital lease obligation	(649)	(483)	(371)
Net cash provided by (used in) financing activities	1,414	787	(31,572)
Net increase in cash and cash equivalents	264	11,868	409
Cash and cash equivalents at beginning of year	26,052	14,184	13,775
Cash and cash equivalents at end of year	26,316	26,052	14,184
Supplemental cash flow information:
Cash paid for interest	81	85	70
Cash paid for income taxes	59	472	220
Supplemental non-cash investing and financing activities:
Assets acquired under capital lease	$ 738	$ 610	$ 365