Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (32,506)	$ (25,771)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation, amortization and write-off of property and equipment	1,554	1,430
Amortization and write-off of intangibles	589	996
Stock-based compensation	7,298	6,757
Changes in operating assets and liabilities:
Trade accounts receivable	2,516	(1,326)
Other current assets	92	(476)
Other assets	(57)	5
Accounts payable and other accrued liabilities	(878)	585
Deferred revenue	182	189
Net cash used in operating activities	(21,210)	(17,611)
Cash flows from investing activities:
Purchase of property and equipment	(1,292)	(2,188)
Capitalized patent costs	(747)	(819)
Maturity of marketable securities	34,558	60,360
Purchase of marketable securities	(24,021)	(38,387)
Net cash provided by investing activities	8,498	18,966
Cash flows from financing activities:
Issuance of common stock, net of issuance costs	0	29,677
Exercise of stock options	1,256	1,183
Purchase of common stock	(2,089)	(3,030)
Net cash provided by (used in) financing activities	(833)	27,830
Net increase (decrease) in cash and cash equivalents	(13,545)	29,185
Cash and cash equivalents at beginning of period	40,823	11,638
Cash and cash equivalents at end of period	27,278	40,823
Supplemental disclosure of cash flow information:
Cash received (paid) for income taxes, net	88	(37)
Supplemental schedule of non-cash investing activities:
Property and equipment and patent costs in accounts payable	(79)	(153)
Stock-based compensation capitalized to software and patent costs	$ 170	$ 197