Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ (14,710)	$ (16,711)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	108	117
Stock-based compensation expense	2,331	2,000
Changes in operating assets and liabilities:
Prepaid expenses and other assets	1,470	267
Accounts receivable	(375)	162
Accounts payable	134	(553)
Accrued expenses and other liabilities	(511)	575
Net cash used in operating activities	(11,553)	(14,143)
Cash flows from investing activities:
Purchases of property and equipment	(11)
Net cash used in investing activities	(11)
Cash flows from financing activities:
Proceeds from stock options exercised	244	86
Proceeds from public offering, net of underwriters' discounts	33,672
Payment of offering costs	(181)
Net cash provided by financing activities	33,735	86
Net increase (decrease) in cash, cash equivalents and restricted cash	22,171	(14,057)
Cash, cash equivalents and restricted cash, at beginning of period	99,194	151,128
Cash, cash equivalents and restricted cash, at end of period	121,365	$ 137,071
Supplemental disclosure of non-cash investing and financing activities:
Offering costs not yet paid	148
Property and equipment in accounts payable	$ 25