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Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses, Allowance for Credit Losses and Fair Value of Debt Securities Available-For-Sale (Detail) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 516.3	$ 612.4
Unrealized Gain	0.6	1.5
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0	0.0
Fair Value	516.9	613.9
Short-term investments | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	212.0	299.3
Unrealized Gain	0.5	1.4
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0	0.0
Fair Value	212.5	300.7
Short-term investments | Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	135.7	230.9
Unrealized Gain	0.1	0.1
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0	0.0
Fair Value	135.8	231.0
Short-term investments | Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	168.6	82.2
Unrealized Gain	0.0	0.0
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0	0.0
Fair Value	168.6	82.2
Long-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	337.9	226.7
Unrealized Gain	0.2	0.5
Unrealized Loss	(0.3)	(0.1)
Allowance for Credit Losses	0.0	0.0
Fair Value	337.8	227.1
Long-term investments | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	230.3	144.8
Unrealized Gain	0.1	0.4
Unrealized Loss	(0.2)	0.0
Allowance for Credit Losses	0.0	0.0
Fair Value	230.2	145.2
Long-term investments | Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	107.6	81.9
Unrealized Gain	0.1	0.1
Unrealized Loss	(0.1)	(0.1)
Allowance for Credit Losses	0.0	0.0
Fair Value	$ 107.6	$ 81.9

X

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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