Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flow from operating activities
Net loss	$ (17,670)	$ (12,954)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,674	656
Depreciation and amortization	953	828
Share of loss from equity method investee	431	226
Amortization of premiums on investments	(747)	0
Other non-operating income	0	(59)
Changes in operating assets and liabilities
Amounts owed by a related party	83	(29)
Other long-term assets	(30)	0
Prepaid expenses and other current assets	(759)	(693)
Accounts payable	(517)	(306)
Accrued liabilities	1,606	410
Other current liabilities	14	9
Deferred revenue	(1,750)	(1,250)
Deferred rent	(127)	(85)
Net cash used in operating activities	(16,839)	(13,247)
Cash flow from investing activities
Purchases of short-term and long-term investments	(72,407)	(41,084)
Proceeds from maturities of short-term and long-term investments	87,314	25,647
Purchases of property and equipment	(628)	(1,054)
Net cash provided by (used in) investing activities	14,279	(16,491)
Cash flow from financing activities
Repurchase of unvested shares of stock	(5)	0
Proceeds from initial public offering, net of issuance costs	0	126,259
Proceeds from issuance of common stock upon exercise of stock options, net of repurchases	0	3,304
Payment of preferred stock issuance costs	0	(135)
Net cash provided by financing activities	(5)	129,428
Net decrease (increase) in cash and cash equivalents	(2,565)	99,690
Cash, cash equivalents and restricted cash at beginning of period	71,267	98,629
Cash, cash equivalents and restricted cash at end of period	68,702	198,319
Non-cash investing and financing activities:
Unpaid financing cost included in accounts payable and accrued liabilities	0	1,170
Unpaid portion of property and equipment purchases included in accounts payable and accrued liabilities	380	695
Vesting of early exercised stock options and restricted stock	$ 273	$ 278