Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flow from operating activities
Net loss	$ (72,827)	$ (45,760)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	7,932	3,804
Depreciation and amortization	1,629	1,836
Share of loss from equity method investee	613	844
Gain on deemed sale from equity method investee	(613)
Amortization of premiums on investments	(323)	(1,580)
Changes in operating assets and liabilities:
Receivable from collaboration partners	(90)
Amounts owed by a related party		21
Prepaid expenses and other current assets	(4,601)	(952)
Other long-term assets	(135)	(35)
Accounts payable	6,668	(566)
Accrued research and development expenses	9,327	10,094
Accrued liabilities	661	(565)
Other current liabilities		29
Deferred revenue	(3,500)	(3,500)
Deferred rent	(266)	(262)
Other long-term liabilities	310
Net cash used in operating activities	(55,215)	(36,592)
Cash flow from investing activities
Purchases of short-term and long-term investments	(139,689)	(123,837)
Proceeds from maturities of short-term and long-term investments	92,134	165,968
Sales of short-term investments	1,003
Purchases of property and equipment	(465)	(1,179)
Net cash (used in) provided by investing activities	(47,017)	40,952
Cash flow from financing activities
Proceeds from issuance of common stock, net of issuance costs	326,512
Proceeds from issuance of common stock pursuant to equity award plans	3,001	695
Repurchase of unvested shares of stock	(33)	(51)
Net cash provided by financing activities	329,480	644
Net increase in cash and cash equivalents	227,248	5,004
Cash, cash equivalents and restricted cash at beginning of period	58,140	71,267
Cash, cash equivalents and restricted cash at end of period	285,388	76,271
Non-cash investing and financing activities:
Unpaid portion of financing costs included in accounts payable	266
Unpaid portion of deferred financing costs included in accounts payable	316
Unpaid portion of property and equipment purchases included in accounts payable and accrued liabilities	110	48
Vesting of early exercised stock options and restricted stock	$ 436	$ 533