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	$ (18,898)	$ (28,643)	$ (24,850)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	126	23	11
Stock-based compensation expense	2,880	3,077	2,108
Amortization of debt issuance costs	4	7	7
Amortization of discount on investments	(28)
Amortization of discount on investments			2
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(779)	1,396	(947)
Accounts payable and accrued expenses	(2,121)	(629)	3,540
Net cash used in operating activities	(18,816)	(24,769)	(20,129)
Cash flows from investing activities
Purchases of investments	(24,892)	(2,434)	(7,705)
Maturities of short-term investments	3,922	4,474	1,743
Purchases of property and equipment	(450)	(644)	(352)
Net cash (used in) provided by investing activities	(21,420)	1,396	(6,314)
Cash flows from financing activities
Proceeds from exercise of stock options	96	124	373
Principal payments of notes payable	(5,247)	(2,128)
Principal payments of notes payable			28,278
Proceeds from equity offerings, net of offering costs	52,547	(167)	(206)
Proceeds from short-term bank borrowings	193
Net cash provided by (used in) financing activities	47,589	(2,171)	28,445
Net increase (decrease) in cash and cash equivalents	7,353	(25,544)	2,002
Cash and cash equivalents-beginning of year	26,178	51,722	49,720
Cash and cash equivalents-end of year	33,531	26,178	51,722
Supplemental disclosure of cash flow information
Cash paid for interest	$ 184	460	468
Financing arrangement with third-party vendor			584
Property and equipment in accounts payable		$ 134
Accrued equity offering costs			$ 142