Debt Securities - Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 429.7
Less Than 12 Months, Unrealized Loss	(3.2)
Total Fair Value	429.7
Total Unrealized Loss	(3.2)
Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Fair Value	19.9
Less Than 12 Months, Unrealized Loss	(0.1)
Total Fair Value	19.9
Total Unrealized Loss	(0.1)
Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Fair Value	409.8	$ 186.1
Less Than 12 Months, Unrealized Loss	(3.1)	(0.1)
12 Months or Longer, Fair Value		0.0
12 Months or Longer, Unrealized Loss		0.0
Total Fair Value	409.8	186.1
Total Unrealized Loss	$ (3.1)	$ (0.1)