Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flow from operating activities
Net loss	$ (49,594)	$ (53,082)	$ (17,970)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	3,874	495	90
Depreciation and amortization	3,664	2,612	1,314
Share of loss from equity method investee	886	416	0
Gain on deemed sale from equity method investee	(1,229)	0	0
(Accretion of discounts) amortization of premiums on investments	(1,752)	0	0
Other non-operating income	(177)	(98)	0
Changes in operating assets and liabilities
Amounts owed by a related party	(58)	380	0
Prepaid expenses and other current assets	(1,180)	(751)	(602)
Other long-term assets	(80)	(6)	(200)
Accounts payable	(69)	(267)	3,569
Accrued liabilities	3,497	1,582	808
Other current liabilities	43	(136)	647
Deferred revenue	(353)	23,587	0
Deferred rent	(468)	209	(600)
Net cash used in operating activities	(42,996)	(25,059)	(12,944)
Cash flow from investing activities
Purchases of short-term and long-term investments	(261,552)	(96,830)	(33,762)
Proceeds from maturities of short-term and long-term investments	151,855	53,273	0
Purchases of property and equipment	(3,743)	(5,514)	(4,099)
Investment in related party	0	0	(1,000)
Net cash used in investing activities	(113,440)	(49,071)	(38,861)
Cash flow from financing activities
Proceeds from initial public offering, net of issuance costs	125,111	0	0
Proceeds from issuance of preferred stock, net of issuance costs	0	106,877	69,817
Proceeds from issuance of common stock upon exercise of stock options, net of repurchases	4,098	892	283
Deferred initial public offering costs	0	(373)	0
Payment of preferred stock issuance costs	(135)	0	0
Net cash provided by financing activities	129,074	107,396	70,100
Net decrease in cash and cash equivalents	(27,362)	33,266	18,295
Cash and cash equivalents at beginning of period	98,426	65,160	46,865
Cash and cash equivalents at end of period	71,064	98,426	65,160
Non-cash investing and financing activities:
Unpaid financing cost included in accounts payable and accrued liabilities	0	1,058	0
Unpaid portion of property and equipment purchases included in accounts payable and accrued liabilities	136	338	618
Vesting of early exercised stock options and restricted stock	$ 1,276	$ 235	$ 63