Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating activities
Net loss	$ (56,763)	$ (38,992)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,109	2,403
Stock–based compensation expense	10,785	6,874
Net amortization of discounts on marketable securities	(607)	(1,317)
Non-cash operating lease expense	321	1,265
Other non-cash items	(53)	0
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(2,454)	2,849
Accounts payable	(292)	1,965
Accruals and other current liabilities	(3,827)	(4,593)
Contract liabilities	(3,069)	(777)
Operating lease liability	(642)	0
Net cash used in operating activities	(54,492)	(30,323)
Investing activities
Purchases of marketable securities	(153,155)	(109,810)
Purchases of property and equipment	(879)	(2,028)
Maturities and sales of marketable securities	140,701	103,791
Net cash used in investing activities	(13,333)	(8,047)
Financing activities
Proceeds from public offering of common stock, net of issuance costs	193,948	0
Proceeds from exercise of awards under equity incentive plans	1,965	908
Net cash provided by financing activities	195,913	908
Net increase (decrease) in cash, cash equivalents and restricted cash	128,088	(37,462)
Cash, cash equivalents and restricted cash at beginning of period	80,949	78,623
Cash, cash equivalents and restricted cash at end of period	209,037	41,161
Supplemental disclosures of cash flow information
Tenant improvements provided by the landlord	0	11,341
Property and equipment purchases accrued but not yet paid	$ 0	$ 1,569