Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended				12 Months Ended			134 Months Ended
	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018
Operating activities:
Net loss	$ (6,445)	$ (5,169)	$ (7,976)	$ (9,021)	$ (24,871)	$ (30,784)	$ (71,865)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization					562	645	490
Stock-based compensation					3,232	4,367	6,387
Noncash interest expense on loans					331	786	1,050
Impairment of property and equipment					108			$ 0
Loss on disposal of assets					12	177	70
Revaluation of warrant liability					4	(9)	(65)
(Accretion) amortization of bond (discount) premium on marketable securities					(484)	(182)	474
Change in right-of-use asset					301
Changes in operating assets and liabilities:
Prepaid expenses and other assets					(1,122)	786	612
Operating lease liabilities					(482)
Accounts payable, accrued expenses and other liabilities					(1,258)	(1,161)	(8,408)
Net cash used in operating activities					(23,667)	(25,375)	(71,255)
Investing activities:
Purchases of property and equipment						(169)	(3,495)
Maturities of marketable securities					79,389	68,982	99,018
Purchases of marketable securities					(53,806)	(47,813)	(62,743)
Net cash provided by investing activities					25,583	21,000	32,780
Financing activities:
Proceeds from exercise of common stock options					28	83	361
Proceeds from issuance of common stock, net					10,312	33,195	20,712
Payment of employee withholding taxes on vested restricted stock units					(158)
Capital lease payments					(12)
Capital lease payments						(11)	(8)
Proceeds from loans payable, net							9,921
Repayments of loans payable, net					(12,667)	(12,667)
Net cash (used in) provided by financing activities					(2,497)	20,600	30,986
Net (decrease) increase in cash, cash equivalents and restricted cash					(581)	16,225	(7,489)
Cash, cash equivalents and restricted cash - beginning of period		$ 34,195		$ 17,970	34,195	17,970	25,459
Cash, cash equivalents and restricted cash - end of period	$ 33,614		$ 34,195		33,614	34,195	17,970	$ 34,195
Supplemental disclosure of cash flow information:
Cash paid for interest					600	$ 1,446	1,730
Capital lease additions							27
Fair value of common stock warrants issued to underwriters					$ 347		$ 501