Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (29,810)	$ (46,040)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	277	239
Stock-based compensation	5,716	6,510
Accretion/amortization of investments, net	61	(1,926)
Non-cash interest expense	0	332
Changes in operating assets and liabilities:
Receivable from collaborative partners	0	(5,000)
Prepaid expenses and other assets	(2,478)	1,609
Accounts payable and other liabilities	(8,488)	12,660
Net cash used in operating activities	(34,722)	(31,616)
CASH FLOWS FROM INVESTING ACTIVITIES:
Acquisition of investments	(86,947)	(132,313)
Sales and maturities of investments	173,262	200,267
Purchases of property and equipment	(170)	(421)
Net cash provided by investing activities	86,145	67,533
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, upon the exercise of stock options	107	789
Payments on notes payable	(1,375)	(3,750)
Net cash used in financing activities	(1,268)	(2,961)
Net increase in cash, cash equivalents, and restricted cash	50,155	32,956
Cash, cash equivalents and restricted cash, beginning of period	171,077	113,656
Cash, cash equivalents and restricted cash, end of period	221,232	146,612
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION
Interest paid	4	301
Non-cash investing and financing activities:
Amounts accrued for property and equipment	$ 25	$ 14