Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flow from operating activities
Net loss	$ (45,760)	$ (26,488)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	3,804	1,772
Depreciation and amortization	1,836	1,743
Share of loss from equity method investee	844	377
Gain on deemed sale from equity method investee	0	(1,229)
Amortization of premiums on investments	(1,580)	0
Other non-operating income	0	(177)
Changes in operating assets and liabilities:
Amounts owed by a related party	21	(168)
Other long-term assets	(35)	0
Prepaid expenses and other current assets	(952)	(409)
Accounts payable	(566)	1,347
Accrued liabilities	9,529	1,812
Other current liabilities	29	20
Deferred revenue	(3,500)	(2,500)
Deferred rent	(262)	(224)
Net cash used in operating activities	(36,592)	(24,124)
Cash flow from investing activities
Purchases of short-term and long-term investments	(123,837)	(90,671)
Proceeds from maturities of short-term and long-term investments	165,968	60,415
Purchases of property and equipment	(1,179)	(2,358)
Net cash provided by (used in) investing activities	40,952	(32,614)
Cash flow from financing activities
Repurchase of unvested shares of stock	(51)	0
Proceeds from initial public offering, net of issuance costs	0	125,114
Proceeds from issuance of common stock upon exercise of stock options, net of repurchases	695	3,331
Payment of preferred stock issuance costs	0	(135)
Net cash provided by financing activities	644	128,310
Net decrease (increase) in cash and cash equivalents	5,004	71,572
Cash, cash equivalents and restricted cash at beginning of period	71,267	98,629
Cash, cash equivalents and restricted cash at end of period	76,271	170,201
Non-cash investing and financing activities:
Unpaid portion of property and equipment purchases included in accounts payable and accrued liabilities	48	1,007
Vesting of early exercised stock options and restricted stock	$ 533	$ 647