Debt Securities - Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	$ 82.0
Less Than 12 Months, Unrealized Loss	(0.4)
12 Months or Longer, Fair Value	0.0
12 Months or Longer, Unrealized Loss	0.0
Total, Fair Value	82.0
Total, Unrealized Loss	(0.4)
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	464.0	$ 428.6
Less Than 12 Months, Unrealized Loss	(5.9)	(1.6)
12 Months or Longer, Fair Value	18.8	0.0
12 Months or Longer, Unrealized Loss	(0.4)	0.0
Total, Fair Value	482.8	428.6
Total, Unrealized Loss	(6.3)	(1.6)
Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	326.1	230.5
Less Than 12 Months, Unrealized Loss	(5.8)	(1.0)
12 Months or Longer, Fair Value	10.7	0.0
12 Months or Longer, Unrealized Loss	(0.3)	0.0
Total, Fair Value	336.8	230.5
Total, Unrealized Loss	$ (6.1)	$ (1.0)