CONSOLIDATED 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	$ (36,726)	$ (18,898)	$ (28,643)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	127	126	23
Stock-based compensation expense	4,788	2,880	3,077
Amortization of debt issuance costs		4	7
Amortization of discount on investments	(79)	(28)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(237)	(779)	1,396
Accounts payable and accrued expenses	4,285	(2,121)	(629)
Net cash used in operating activities	(27,842)	(18,816)	(24,769)
Cash flows from investing activities
Purchases of investments		(24,892)	(2,434)
Maturities of short-term investments	20,000	3,922	4,474
Purchases of property and equipment	(83)	(450)	(644)
Net cash provided by (used in) investing activities	19,917	(21,420)	1,396
Cash flows from financing activities
Proceeds from exercise of stock options	14	96	124
Principal payments of notes payable		(5,247)	(2,128)
Proceeds from equity offerings, net of offering costs	42,300	52,547	(167)
Proceeds from short-term bank borrowings		193
Repayments of short-term bank borrowings	(193)
Net cash provided by (used in) financing activities	42,121	47,589	(2,171)
Net increase (decrease) in cash and cash equivalents	34,196	7,353	(25,544)
Cash and cash equivalents—beginning of year	33,531	26,178	51,722
Cash and cash equivalents—end of year	67,727	33,531	26,178
Supplemental disclosure of cash flow information
Cash paid for interest		$ 184	460
Accrued equity offering costs	$ 153
Property and equipment in accounts payable			$ 134