Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities:
Net loss	$ (22,996)	$ (8,558)	$ (7,581)
Adjustments to reconcile net loss to net cash used in operating activities:
Amortization of intangible assets	0	2,812	3,374
Depreciation and amortization	1,042	1,734	2,035
Stock-based compensation	7,091	5,673	5,126
Loss (gain) on disposal of property and equipment	9	(42)	32
Gain from extinguishment of asset retirement obligation	(207)	0	0
Income tax benefit related to marketable securities	0	0	(314)
Changes in operating assets and liabilities:
Accounts receivable, net	(5,651)	1,405	(3,459)
Inventories	(210)	167	403
Prepaid expenses and other current assets	157	7	10
Restricted cash	(8)	(841)	0
Other assets	(44)	52	(16)
Accounts payable	(801)	942	(1,274)
Accrued compensation	439	983	385
Other accrued liabilities	1,399	(593)	(1,062)
Deferred revenue	11,017	(6,442)	1,908
Net cash used in operating activities	(8,763)	(2,701)	(433)
Investing activities:
Purchase of property and equipment	(985)	(888)	(1,199)
Proceeds from disposal of property and equipment	2	42	18
Changes in restricted cash	75	4	(76)
Net cash used in investing activities	(908)	(842)	(1,257)
Financing activities:
Proceeds from exercises of stock options	266	1,034	289
Proceeds from issuance of common stock in connection with public offering, net of underwriting discounts and commission	23,782	0	0
Costs incurred in connection with public offering	(553)	0	0
Principal payments on capital lease obligations	(175)	0	0
Taxes paid related to net share settlement of equity awards	(1,670)	(1,524)	(1,813)
Net cash provided by (used in) financing activities	21,650	(490)	(1,524)
Net increase (decrease) in cash and cash equivalents	11,979	(4,033)	(3,214)
Cash and cash equivalents at the beginning of the year	19,240	23,273	26,487
Cash and cash equivalents at the end of the year	31,219	19,240	23,273
Supplemental disclosure of cash flow information:
Interest paid	141	14	0
Income taxes	32	5	8
Supplemental non-cash financing activities:
Equipment acquired under capital leases	$ 862	$ 0	$ 0