Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Cash flow from operating activities
Net loss	$ (84,710)	$ (49,594)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	8,981	3,874
Depreciation and amortization	3,578	3,664
Share of loss from equity method investee	1,202	886
Gain on deemed sale from equity method investee	0	(1,229)
Amortization of premiums on investments	(2,638)	(1,752)
Other non-operating income	0	(177)
Changes in operating assets and liabilities:
Receivable from collaboration partner	(132)	0
Amounts owed by a related party	83	(58)
Prepaid expenses and other current assets	(1,982)	(1,180)
Other long-term assets	(588)	(80)
Accounts payable	1,726	(69)
Accrued liabilities	3,499	3,497
Other current liabilities	57	43
Deferred revenue	(2,000)	(353)
Deferred rent	(538)	(468)
Net cash used in operating activities	(73,462)	(42,996)
Cash flow from investing activities
Purchases of short-term and long-term investments	(247,755)	(261,552)
Proceeds from maturities of short-term and long-term investments	308,892	151,855
Purchases of property and equipment	(1,925)	(3,743)
Net cash provided by (used in) investing activities	59,212	(113,440)
Cash flow from financing activities
Proceeds from initial public offering, net of issuance costs	0	125,111
Proceeds from issuance of common stock	1,217	4,098
Repurchase of common stock issued under early exercised options	(94)	0
Payment of preferred stock issuance costs	0	(135)
Net cash provided by financing activities	1,123	129,074
Net decrease in cash, cash equivalents and restricted cash	(13,127)	(27,362)
Cash, cash equivalents and restricted cash at beginning of period	71,267	98,629
Cash, cash equivalents and restricted cash at end of period	58,140	71,267
Non-cash investing and financing activities:
Unpaid portion of property and equipment purchases included in accounts payable and accrued liabilities	12	136
Vesting of early exercised stock options and restricted stock	$ 1,029	$ 1,276