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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	Sep. 30, 2021	Dec. 31, 2020
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 454.6	$ 612.4
Unrealized Gain	0.2	1.5
Unrealized Loss	0.0	0.0
Fair Value	454.8	613.9
Short-term investments | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	170.6	299.3
Unrealized Gain	0.2	1.4
Unrealized Loss	0.0	0.0
Fair Value	170.8	300.7
Short-term investments | Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	75.4	230.9
Unrealized Gain	0.0	0.1
Unrealized Loss	0.0	0.0
Fair Value	75.4	231.0
Short-term investments | Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	208.6	82.2
Unrealized Gain	0.0	0.0
Unrealized Loss	0.0	0.0
Fair Value	208.6	82.2
Long-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	513.8	226.7
Unrealized Gain	0.3	0.5
Unrealized Loss	(0.4)	(0.1)
Fair Value	513.7	227.1
Long-term investments | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	328.6	144.8
Unrealized Gain	0.2	0.4
Unrealized Loss	(0.3)	0.0
Fair Value	328.5	145.2
Long-term investments | Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	185.2	81.9
Unrealized Gain	0.1	0.1
Unrealized Loss	(0.1)	(0.1)
Fair Value	$ 185.2	$ 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).

+ References

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Amount, before tax, of unrealized loss 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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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of 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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