Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Operating Activities
Net loss	$ (108,635)	$ (101,283)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,841	2,932
Stock-based compensation	21,805	17,310
Other non-cash items	1,552	(75)
Changes in assets and liabilities:
Prepaid expenses and other assets	1,231	(3,731)
Collaboration receivables (all amounts are with a related party)	(14,014)	164
Non-cash lease expense	4,249	3,804
Accounts payable	6,315	1,435
Accrued expenses	(3,025)	5,132
Operating lease obligations	(4,533)	(3,214)
Other changes in operating assets and liabilities	17	(27)
Net cash used in operating activities	(92,197)	(77,553)
Investing Activities
Purchases of investments	(161,741)	(379,096)
Proceeds from sales and maturities of investments	220,240	228,258
Purchases of property and equipment	(3,303)	(2,166)
Net cash provided by (used in) investing activities	55,196	(153,004)
Financing Activities
Proceeds from issuance of common stock from public offering, net of issuance costs	492,414	248,130
Net proceeds from exercises and vesting of stock awards, and ESPP contributions	36,595	7,917
Net cash provided by financing activities	529,009	256,047
Net increase in cash, cash equivalents and restricted cash	492,008	25,490
Cash, cash equivalents and restricted cash at beginning of period	239,274	145,649
Cash, cash equivalents and restricted cash at end of period	731,282	171,139
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Purchase of property and equipment included in accounts payable and accrued expenses	281	512
Reclassification of warrant liability to additional paid-in capital	$ 3,729	$ 0