Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Operating activities
Net loss	$ (35,235)	$ (73,395)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	468	422
Stock-based compensation expense	5,513	7,483
Net accretion and amortization of investments in marketable securities	(240)	(1,909)
Changes in assets and liabilities:
Interest receivable and other current assets	42	(205)
Prepaid expenses	7,071	(6,074)
Other assets		1,019
Accounts payable	(1,556)	(479)
Accrued restructuring charges	(3,100)
Accrued liabilities	(2,872)	3,363
Net cash used in operating activities	(29,909)	(69,775)
Investing activities
Purchases of property and equipment	(21)	(289)
Purchases of marketable securities	(153,951)	(246,180)
Proceeds from maturities of marketable securities	206,278	198,881
Proceeds from sale of marketable securities		3,980
Net cash provided by (used in) investing activities	52,306	(43,608)
Financing activities
Proceeds from issuance of common stock, net of issuance costs		107,746
Proceeds from issuance of common stock pursuant to equity award plans	7	104
Cash provided by financing activities	7	107,850
Net increase (decrease) in cash and cash equivalents	22,404	(5,533)
Cash and cash equivalents at beginning of period	24,869	48,995
Cash and cash equivalents at end of period	47,273	43,462
Supplemental disclosure
Cash paid for amounts included in the measurement of lease liabilities	$ 484	$ 470