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 loss	$ (25,771)	$ (21,672)	$ (17,934)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation, amortization and write-off of property and equipment	1,430	1,336	1,237
Amortization and write-off of intangibles	996	1,121	1,196
Changes in allowance for doubtful accounts	0	0	(7)
Stock-based compensation	6,757	5,553	5,077
Changes in operating assets and liabilities:
Trade accounts receivable	(1,326)	(462)	(64)
Other current assets	(476)	(208)	1,124
Other assets	5	7	40
Accounts payable and other accrued liabilities	585	637	125
Deferred revenue	189	(176)	(534)
Net cash used in operating activities	(17,611)	(13,864)	(9,740)
Cash flows from investing activities:
Purchase of property and equipment	(2,188)	(1,837)	(1,218)
Capitalized patent costs	(819)	(790)	(895)
Maturity of marketable securities	60,360	41,052	42,264
Purchase of marketable securities	(38,387)	(53,914)	(45,340)
Net cash provided by (used in) investing activities	18,966	(15,489)	(5,189)
Cash flows from financing activities:
Issuance of common stock, net of issuance costs	29,677	39,700	12,896
Exercise of stock options	1,183	668	1,514
Purchase of common stock	(3,030)	(2,537)	(2,443)
Net cash provided by financing activities	27,830	37,831	11,967
Net increase (decrease) in cash and cash equivalents	29,185	8,478	(2,962)
Cash and cash equivalents at beginning of period	11,638	3,160	6,122
Cash and cash equivalents at end of period	40,823	11,638	3,160
Supplemental disclosure of cash flow information:
Cash received (paid) for income taxes, net	(37)	(38)	1,233
Supplemental schedule of non-cash investing activities:
Property and equipment and patent costs in accounts payable	(153)	35	73
Stock-based compensation capitalized to software and patent costs	$ 197	$ 164	$ 174