Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (53,577)	$ (77,073)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	415	374
Stock-based compensation	8,521	9,654
Accretion/amortization of investments, net	239	(2,449)
Non-cash interest expense	0	503
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(2,902)	2,099
Accounts payable and other liabilities	(4,431)	13,180
Net cash used in operating activities	(51,735)	(53,712)
CASH FLOWS FROM INVESTING ACTIVITIES:
Acquisition of investments	(156,572)	(175,049)
Sales and maturities of investments	247,707	303,146
Purchases of property and equipment	(258)	(701)
Net cash provided by investing activities	90,877	127,396
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, upon the exercise of stock options	370	1,243
Payments on notes payable	(1,375)	(5,625)
Net cash used in financing activities	(1,005)	(4,382)
Net increase in cash, cash equivalents, and restricted cash	38,137	69,302
Cash, cash equivalents and restricted cash, beginning of period	171,077	113,656
Cash, cash equivalents and restricted cash, end of period	209,214	182,958
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION
Interest paid	4	384
Non-cash investing and financing activities:
Amounts accrued for property and equipment	$ 165	$ 82