Marketable Securities and Fair Value Measurements (Details) - USD ($) $ in Millions	Dec. 31, 2021	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Nonrecurring | Pfizer, Inc. | Stock Purchase Agreement
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Non-recurring fair value measurements		$ 264.6
Level 2 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 1,399.0		$ 99.6
Gross Unrealized Gains	0.0		0.6
Gross Unrealized Losses	(4.7)		0.0
Fair Value	1,394.3		100.2
Level 2 | Fair Value, Measurements, Recurring | Corporate Bonds Maturing 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	784.0
Gross Unrealized Gains	0.0
Gross Unrealized Losses	(0.7)
Fair Value	783.3
Level 2 | Fair Value, Measurements, Recurring | Corporate Bonds Maturing 2023 -2024
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	582.6
Gross Unrealized Gains	0.0
Gross Unrealized Losses	(3.9)
Fair Value	578.7
Level 2 | Fair Value, Measurements, Recurring | Government Securities Maturing 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	32.4
Gross Unrealized Gains	0.0
Gross Unrealized Losses	(0.1)
Fair Value	$ 32.3
Level 2 | Fair Value, Measurements, Recurring | Corporate Bonds Maturing 2021
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost			99.6
Gross Unrealized Gains			0.6
Gross Unrealized Losses			0.0
Fair Value			$ 100.2