Debt Securities - Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	$ 164.6
Less Than 12 Months, Unrealized Loss	(0.2)
12 Months or Longer, Fair Value	0.0
12 Months or Longer, Unrealized Loss	0.0
Total Fair Value	164.6
Total Unrealized Loss	$ (0.2)
Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value		$ 95.0
Less Than 12 Months, Unrealized Loss		(0.1)
12 Months or Longer, Fair Value		0.0
12 Months or Longer, Unrealized Loss		0.0
Total Fair Value		95.0
Total Unrealized Loss		$ (0.1)