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	$ (12,960)	$ (8,973)	$ (9,238)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	793	1,117	1,159
Write-off of abandoned patents and licensed technology	0	374	230
Gain on sale of property and equipment	0	0	1
Compensation expense related to stock options and awards	756	990	1,902
Purchase of technology license with common stock	0	0	775
Changes in operating assets and liabilities:
Deferred contract costs	4,989	5,428	(817)
Receivables and other assets	1,973	(840)	995
Accounts payable and accrued expenses	1,313	557	(1,415)
Deferred revenue	8,682	2,768	250
Deferred rent	223	633	432
Net cash used in operating activities	(8,601)	(8,388)	(6,948)
Cash flows from investing activities:
Maturities of marketable securities	31,429	30,652	20,183
Purchases of marketable securities	36,640	38,069	20,050
Purchases of property and equipment	80	255	72
Proceeds from the sale of property and equipment	0	0	3
Patent and licensed technology expenditures	0	0	53
Net cash (used in) provided by investing activities	(5,291)	(7,672)	11
Cash flows from financing activities:
Net proceeds from issuance of common stock and pre-funded warrants	30,549	7,758	3
Payment of withholding taxes for net settlement of restricted stock units	4	14	23
Net cash provided by (used in) financing activities	30,545	7,744	(20)
Net increase (decrease) in cash, cash equivalents and restricted cash	16,653	(8,316)	(6,957)
Cash, cash equivalents and restricted cash at beginning of year	8,380	16,696	23,653
Cash, cash equivalents and restricted cash at end of year	$ 25,033	$ 8,380	$ 16,696