Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flow from operating activities
Net loss	$ (61,296)	$ (63,035)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	11,637	14,348
Depreciation	1,669	1,321
Unrealized gain on corporate equity securities	(14,525)
Other non-cash items, net	(289)	(1,959)
Changes in operating assets and liabilities:
Accounts receivable	(1,447)	18
Prepaid expenses and other current assets	(3,254)	301
Right-of-use assets	192	1,984
Other non-current assets	93
Accounts payable	1,026	(1,459)
Accrued expenses	(12,439)	(4,627)
Deferred revenue	(7,375)	621
Operating lease liabilities	(278)	(2,325)
Other current and non-current liabilities	1,344	3
Net cash used in operating activities	(84,942)	(54,809)
Cash flow from investing activities
Purchases of property and equipment	(3,993)	(2,231)
Proceeds from the sale of equipment	14	36
Purchases of marketable securities	(66,384)	(106,569)
Proceeds from maturities of marketable securities	191,000	235,500
Net cash provided by investing activities	120,637	126,736
Cash flow from financing activities
Proceeds from offering of common stock, net of issuance costs	203,964
Proceeds from exercise of stock options	9,886	3,619
Issuance of common stock under benefit plans	350	283
Net cash provided by financing activities	214,200	3,902
Net increase in cash, cash equivalents, and restricted cash	249,895	75,829
Cash, cash equivalents, and restricted cash, beginning of period	239,802	136,395
Cash, cash equivalents, and restricted cash, end of period	489,697	212,224
Supplemental disclosure of cash and non-cash activities:
Fixed asset additions included in accounts payable and accrued expenses	429	96
Cash paid in connection with operating lease liabilities	5,125	3,089
Offering costs included in accounts payable and accrued expenses	282
Right-of-use assets obtained in exchange of operating lease obligations	$ 3,319	$ 19,461