Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Operating Activities
Net loss	$ (69,391)	$ (55,913)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,938	1,942
Stock-based compensation	13,819	12,004
Other non-cash items	1,563	404
Changes in assets and liabilities:
Prepaid expenses and other assets	(9,293)	(2,508)
Collaboration receivables (all amounts are with a related party)	(31,205)	1,592
Non-cash lease expense	2,792	2,502
Accounts payable	10,334	3,129
Accrued expenses	(6,998)	(1,744)
Operating lease obligations	(2,977)	(1,874)
Other changes in operating assets and liabilities	17	(42)
Net cash used in operating activities	(89,401)	(40,508)
Investing Activities
Purchases of investments	(58,539)	(293,913)
Proceeds from sales and maturities of investments	168,385	104,201
Purchases of property and equipment	(2,420)	(1,273)
Net cash provided by (used in) investing activities	107,426	(190,985)
Financing Activities
Proceeds from issuance of common stock from public offering, net of issuance costs	0	248,130
Net proceeds from exercises and vesting of stock awards, ESPP contributions, and exercise of warrants to purchase common stock	27,819	2,920
Net cash provided by financing activities	27,819	251,050
Net increase in cash, cash equivalents and restricted cash	45,844	19,557
Cash, cash equivalents and restricted cash at beginning of period	239,274	145,649
Cash, cash equivalents and restricted cash at end of period	285,118	165,206
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Purchase of property and equipment included in accounts payable and accrued expenses	352	338
Capitalized follow-on public offering costs included in accrued expenses	$ 423	$ 0