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 loss	$ (16,254)	$ (12,960)	$ (8,973)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	151	793	1,117
Accretion of discount on short-term investments	(249)	0	0
Write-off of abandoned patents	673	0	374
Gain on sale of property and equipment	(2,286)	0	0
Compensation expense related to stock options and awards	361	756	990
Changes in operating assets and liabilities:
Deferred contract costs	0	(4,989)	(5,428)
Receivables and other assets	5,064	(1,973)	840
Accounts payable and accrued expenses	(1,666)	1,313	557
Deferred revenue	(239)	8,682	2,768
Deferred rent	0	(223)	(633)
Net cash used in operating activities	(14,445)	(8,601)	(8,388)
Cash flows from investing activities:
Maturities of marketable securities	44,688	31,429	30,652
Purchases of marketable securities	(45,009)	(36,640)	(38,069)
Purchases of property and equipment	(36)	(80)	(255)
Proceeds from the sale of property and equipment	1,639	0	0
Net cash provided by (used in) investing activities	1,282	(5,291)	(7,672)
Cash flows from financing activities:
Net proceeds from issuance of common stock and pre-funded warrants	1	30,549	7,758
Payment of withholding taxes for net settlement of restricted stock units	(1)	(4)	(14)
Net cash provided by financing activities	0	30,545	7,744
Net (decrease) increase in cash, cash equivalents and restricted cash	(13,163)	16,653	(8,316)
Cash, cash equivalents and restricted cash at beginning of year	25,033	8,380	16,696
Cash, cash equivalents and restricted cash at end of year	11,870	25,033	8,380
Supplemental disclosure of noncash investing and financing activities:
Gain related to deferred rent	$ 1,067	$ 0	$ 0