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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	Mar. 31, 2021	Dec. 31, 2020
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 520.2	$ 612.4
Unrealized Gain	0.9	1.5
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0	0.0
Fair Value	521.1	613.9
Short-term investments | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	239.6	299.3
Unrealized Gain	0.9	1.4
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0	0.0
Fair Value	240.5	300.7
Short-term investments | Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	187.4	230.9
Unrealized Gain	0.0	0.1
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0	0.0
Fair Value	187.4	231.0
Short-term investments | Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	93.2	82.2
Unrealized Gain	0.0	0.0
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0	0.0
Fair Value	93.2	82.2
Long-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	249.6	226.7
Unrealized Gain	0.2	0.5
Unrealized Loss	(0.2)	(0.1)
Allowance for Credit Losses	0.0	0.0
Fair Value	249.6	227.1
Long-term investments | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	169.0	144.8
Unrealized Gain	0.1	0.4
Unrealized Loss	(0.2)	0.0
Allowance for Credit Losses	0.0	0.0
Fair Value	168.9	145.2
Long-term investments | Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	80.6	81.9
Unrealized Gain	0.1	0.1
Unrealized Loss	0.0	(0.1)
Allowance for Credit Losses	0.0	0.0
Fair Value	$ 80.7	$ 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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