Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income (loss)	$ (30,770)	$ 29,993	$ 2,577
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Stock-based compensation	7,157	5,110	3,713
Depreciation and amortization	1,209	1,016	982
Amortization of contract acquisition costs	1,834
Change in deferred tax assets	17,438	(18,678)	22
Provision for doubtful accounts receivable	(1,257)	1
Other	241	(51)	(7)
Changes in assets and liabilities:
Accounts receivable	(117)	(4,229)	(4,899)
Inventories	768	(13,805)	(2,899)
Other current assets and prepaid expenses	(1,070)	(591)	(432)
Other long-term assets	(2,754)	6	4
Accounts payable	4,277	4,404	639
Accrued liabilities	(3,781)	9,345	3,461
Extended warranty liabilities	3,159
Other long-term liabilities	140		(329)
Deferred revenue	1,305	1,557	(826)
Income tax liability	15	211	(14)
Net cash provided by operating activities	308	14,287	1,992
Cash flows from investing activities:
Acquisition of property, equipment and software*	(1,488)	(855)	(537)
Disposal of property and equipment	41	53	20
Proceeds from sales of marketable investments	13,044	33,640	9,008
Proceeds from maturities of marketable investments	10,050	45,812	25,810
Purchase of marketable investments	(10,874)	(60,956)	(37,693)
Net cash provided by (used in) investing activities	10,773	17,694	(3,392)
Cash flows from financing activities:
Repurchase of common stock		(35,167)	(4,873)
Proceeds from exercise of stock options and employee stock purchase plan	4,399	5,435	10,111
Taxes paid related to net share settlement of equity awards	(3,129)	(1,469)	(618)
Payments on capital lease obligation	(483)	(371)	(313)
Net cash provided by (used in) financing activities	787	(31,572)	4,307
Net increase (decrease) in cash and cash equivalents	11,868	409	2,907
Cash and cash equivalents at beginning of year	14,184	13,775	10,868
Cash and cash equivalents at end of year	26,052	14,184	13,775
Supplemental cash flow information:
Cash paid for interest	85	70	43
Cash paid for income taxes	472	220	222
Supplemental non-cash investing and financing activities:
Assets acquired under capital lease	$ 610	$ 365	$ 801