CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ (16,598)	$ (21,024)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	424	415
Non-cash lease expense	314	287
Net amortization of premiums and discounts on marketable securities	0	(21)
Stock-based compensation expense	1,669	1,729
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(114)	(233)
Other long-term assets	6	(42)
Accounts payable	1,341	0
Accrued expenses and other liabilities	(1,027)	(2,012)
Lease liabilities	(295)	(254)
Net cash used in operating activities	(14,280)	(21,155)
Cash flows from investing activities:
Sales and maturities of marketable securities	0	18,700
Purchases of property and equipment	(9)	(1,026)
Net cash (used in) provided by investing activities	(9)	17,674
Cash flows from financing activities:
Proceeds from exercise of stock options	0	45
Payments for repurchase of common stock for employee tax withholdings	(59)	0
Net cash (used in) provided by financing activities	(59)	45
Net decrease in cash, cash equivalents and restricted cash	(14,348)	(3,436)
Cash, cash equivalents and restricted cash, beginning of period	29,851	53,156
Cash, cash equivalents and restricted cash, end of period	15,503	49,720
Supplemental disclosure of non-cash items:
Purchases of property and equipment included in accounts payable and accrued expenses	$ 0	$ 168