Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net loss	$ (197,614)	$ (36,240)	$ (88,185)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization	7,991	7,415	3,082
Stock-based compensation expense	38,378	18,791	4,409
Net amortization of (discounts) and premiums on marketable securities	(5,019)	(2,705)	753
Non-cash operating lease expense	864	0	0
Other non-cash items	(351)	(36)	1
Changes in operating assets and liabilities:
Prepaid expenses and other assets	6,675	(18,290)	73
Accounts payable	1,033	(526)	207
Accrued and other current liabilities	1,500	9,113	3,578
Contract liability	(6,286)	68,777	0
Non-cash rent expense	0	3,438	0
Operating lease liability	1,253	0	0
Other non-current liabilities	0	379	(553)
Net cash provided by (used in) operating activities	(151,576)	50,116	(76,635)
Investing activities
Purchase of marketable securities	(369,696)	(557,930)	(179,789)
Purchase of property and equipment	(17,919)	(3,393)	(2,875)
Maturities and sales of marketable securities	535,327	273,901	141,498
Net cash provided by (used in) investing activities	147,712	(287,422)	(41,166)
Financing activities
Issuance of common stock in connection with the Takeda Collaboration Agreement	0	94,406	0
Proceeds from exercise of awards under equity incentive plans	6,190	3,999	733
Proceeds from issuance of common stock, net of issuance costs	0	0	265,619
Proceeds from issuance of convertible preferred stock, net of issuance costs	0	0	29,971
Net cash provided by financing activities	6,190	97,019	296,323
Net increase (decrease) in cash, cash equivalents and restricted cash	2,326	(140,287)	178,522
Cash, cash equivalents and restricted cash at beginning of year	78,623	218,910	40,388
Cash, cash equivalents and restricted cash at end of year	80,949	78,623	218,910
Supplemental disclosures of cash flow information
Tenant improvements provided by the landlord	11,343	14,561	0
Property and equipment purchases accrued but not yet paid	0	335	103
Deferred IPO costs incurred but not yet paid	0	0	1,358
Convertible preferred stock issuance costs incurred but not yet paid	0	0	44
Common Stock
Financing activities
Payments of issuance costs related to issuance of stock	0	(1,342)	0
Preferred Stock
Financing activities
Payments of issuance costs related to issuance of stock	$ 0	$ (44)	$ 0