Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Operating activities
Net loss	$ (115,521)	$ (97,329)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	4,254	4,046
Stock–based compensation expense	23,269	20,588
Net amortization of discounts on marketable securities	(1,025)	(2,734)
Non-cash adjustment to operating lease expense	(978)	1,740
Other non-cash items	(39)	(313)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	3,794	2,704
Accounts payable	(744)	1,303
Accruals and other current liabilities	(2,215)	(1,887)
Contract liabilities	(8,644)	(1,616)
Net cash used in operating activities	(97,849)	(73,498)
Investing activities
Purchases of marketable securities	(323,607)	(144,029)
Purchases of property and equipment	(1,194)	(12,299)
Maturities and sales of marketable securities	281,057	212,162
Net cash provided by (used in) investing activities	(43,744)	55,834
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	5,380	3,477
Net cash provided by financing activities	199,328	3,477
Net increase (decrease) in cash, cash equivalents and restricted cash	57,735	(14,187)
Cash, cash equivalents and restricted cash at beginning of period	80,949	78,623
Cash, cash equivalents and restricted cash at end of period	138,684	64,436
Supplemental disclosures of cash flow information
Tenant improvements provided by the landlord	0	11,341
Property and equipment purchases accrued but not yet paid	$ 2	$ 2,162