Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating activities:
Net loss	$ (26,443)	$ (12,377)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,618	2,542
Depreciation expense	938	960
Net amortization of premiums and discounts on investments	(84)	(474)
Other non-cash items	19	0
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(543)	(2,596)
Other non-current assets	18	(103)
Accounts payable	1,407	771
Accrued expenses and other current liabilities	(1,457)	(941)
Deferred revenue—related party	0	(10,981)
Other liabilities	(19)	16
Net cash used in operating activities	(23,546)	(23,183)
Investing activities:
Purchases of investments	(19,856)	(62,019)
Proceeds from maturities of investments	63,650	77,508
Purchases of property and equipment	(50)	(109)
Net cash provided by investing activities	43,744	15,380
Financing activities:
Proceeds from at the market offering, net of issuance costs	1,514	0
Proceeds from exercise of stock options	38	69
Net cash provided by financing activities	1,552	69
Net increase (decrease) in cash, cash equivalents and restricted cash	21,750	(7,734)
Cash, cash equivalents and restricted cash, beginning of period	54,511	49,176
Cash, cash equivalents and restricted cash, end of period	76,261	41,442
Non-cash investing and financing activities:
Purchases of property and equipment in accounts payable and accrued expenses	0	20
Supplemental cash flow information:
Cash paid for lease liabilities	$ 1,090	$ 1,060