Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flow from operating activities
Net loss	$ (27,753)	$ (17,670)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	3,462	1,674
Depreciation and amortization	842	953
Share of loss from equity method investee	482	431
Gain on deemed sale from equity method investee	(482)	0
Amortization of premiums on investments	(248)	(747)
Changes in operating assets and liabilities:
Receivable from collaboration partners	(212)	0
Amounts owed by a related party	0	83
Prepaid expenses and other current assets	(3,704)	(759)
Other long-term assets	(91)	(30)
Accounts payable	(2,342)	(517)
Accrued liabilities	648	1,606
Other current liabilities	0	14
Deferred revenue	(1,750)	(1,750)
Deferred rent	(129)	(127)
Net cash used in operating activities	(31,277)	(16,839)
Cash flow from investing activities
Purchases of short-term and long-term investments	(8,885)	(72,407)
Proceeds from maturities of short-term and long-term investments	49,150	87,314
Purchases of property and equipment	(209)	(628)
Net cash provided by investing activities	40,056	14,279
Cash flow from financing activities
Proceeds from issuance of common stock	643	0
Repurchase of unvested shares of stock	(33)	(5)
Net cash provided by (used in) financing activities	610	(5)
Net increase (decrease) in cash and cash equivalents	9,389	(2,565)
Cash, cash equivalents and restricted cash at beginning of period	58,140	71,267
Cash, cash equivalents and restricted cash at end of period	67,529	68,702
Non-cash investing and financing activities:
Unpaid portion of property and equipment purchases included in accounts payable and accrued liabilities	76	380
Vesting of early exercised stock options and restricted stock	$ 220	$ 273