Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flow from operating activities
Net loss	$ (68,112)	$ (37,299)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,544	2,820
Depreciation and amortization	2,722	2,717
Share of loss from equity method investee	1,202	563
Gain on deemed sale from equity method investee	0	(1,229)
Amortization of premiums on investments	(2,213)	(1,017)
Other non-operating income	0	(177)
Changes in operating assets and liabilities:
Receivable from collaboration partners	(5,276)	(5,000)
Amounts owed by a related party	83	(89)
Other long-term assets	21	0
Prepaid expenses and other current assets	(1,902)	(1,188)
Accounts payable	(94)	(1,029)
Accrued liabilities	3,639	4,141
Other current liabilities	42	33
Deferred revenue	(250)	1,209
Deferred rent	(397)	(344)
Net cash used in operating activities	(63,991)	(35,889)
Cash flow from investing activities
Purchases of short-term and long-term investments	(180,245)	(222,067)
Proceeds from maturities of short-term and long-term investments	253,290	109,601
Purchases of property and equipment	(1,729)	(3,485)
Net cash provided by (used in) investing activities	71,316	(115,951)
Cash flow from financing activities
Repurchase of unvested shares of stock	(94)	0
Proceeds from initial public offering, net of issuance costs	0	125,111
Proceeds from issuance of common stock upon exercise of stock options, net of repurchases	697	3,327
Payment of preferred stock issuance costs	0	(135)
Net cash provided by financing activities	603	128,303
Net increase (decrease) in cash and cash equivalents	7,928	(23,537)
Cash, cash equivalents and restricted cash at beginning of period	71,267	98,629
Cash, cash equivalents and restricted cash at end of period	79,195	75,092
Non-cash investing and financing activities:
Unpaid portion of property and equipment purchases included in accounts payable and accrued liabilities	33	103
Vesting of early exercised stock options and restricted stock	$ 784	$ 982