Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net loss	$ (191.0)	$ (119.3)	$ (70.3)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization	4.8	3.2	1.6
Net accretion of bond discounts/premiums	9.4	1.7	0.2
Forgiveness of debt income	(1.0)	0.0	0.0
Loss (gain) on sale of marketable securities	0.2	(0.4)	0.0
Amortization of right to use assets	1.2	0.9	0.7
Amortization of collaboration contract asset	0.4	0.0	0.0
Stock-based compensation	57.1	30.2	20.1
Changes in operating assets and liabilities:
Accounts receivable	(14.0)	(1.0)	2.8
Other receivables	(3.3)	(1.2)	(4.0)
Prepaid expenses and other current assets	(13.6)	(2.4)	(1.1)
Collaboration contract asset	(12.9)	0.0	0.0
Accounts payable	23.7	2.0	1.5
Accrued expenses	4.2	10.8	3.6
Operating lease liabilities	(1.3)	(0.9)	(0.6)
Deferred revenue	695.5	(13.3)	4.9
Net cash provided by (used in) operating activities	559.4	(89.7)	(40.6)
Cash flows from investing activities:
Purchase of marketable securities	(1,744.6)	(41.2)	(256.5)
Maturities of marketable securities	428.5	174.1	169.7
Sale of marketable securities	7.2	37.8	0.0
Purchase of property, equipment and leasehold improvements	(4.7)	(6.4)	(6.3)
Net cash (used in) provided by investing activities	(1,313.6)	164.3	(93.1)
Cash flows from financing activities:
Repayments of long-term debt	0.0	0.0	(0.2)
Proceeds from issuance of common stock	264.6	460.0	137.7
Payment of common stock offering costs	(4.6)	(27.7)	(0.6)
Proceeds from exercise of stock options	18.6	8.3	2.8
Net cash provided by financing activities	278.6	504.6	139.7
Net (decrease) increase in cash, cash equivalents and restricted cash	(475.6)	579.2	6.0
Cash, cash equivalents and restricted cash, beginning of the period	588.4	9.2	3.2
Cash, cash equivalents and restricted cash, end of the period	112.8	588.4	9.2
Supplemental disclosure of cash flow information:
Purchases of property, equipment and leasehold improvements unpaid at period end	0.5	0.5	0.2
Cash paid for interest	0.1	0.1	0.1
At-the-Market Offering
Payment of common stock offering costs	0.0	(1.6)	0.0
Proceeds from sale of common stock in at-the-market offering	$ 0.0	$ 65.6	$ 0.0