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Debt Securities - Amortized Cost, Gross Unrealized Gain (Loss) Positions and Estimated Fair Value for Available-For-Sale Debt Securities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 612.4	$ 557.3
Unrealized Gain	1.5	0.9
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0
Fair Value	613.9	558.2
Short-term investments | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	299.3	270.5
Unrealized Gain	1.4	0.5
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0
Fair Value	300.7	271.0
Short-term investments | Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	230.9	142.3
Unrealized Gain	0.1	0.4
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0
Fair Value	231.0	142.7
Short-term investments | Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	82.2	144.5
Unrealized Gain	0.0	0.0
Unrealized Loss	0.0	0.0
Allowance for Credit Losses	0.0
Fair Value	82.2	144.5
Long-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	226.7	299.3
Unrealized Gain	0.5	0.5
Unrealized Loss	(0.1)	(0.1)
Allowance for Credit Losses	0.0
Fair Value	227.1	299.7
Long-term investments | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	144.8	250.5
Unrealized Gain	0.4	0.5
Unrealized Loss	0.0	(0.1)
Allowance for Credit Losses	0.0
Fair Value	145.2	250.9
Long-term investments | Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	81.9	48.8
Unrealized Gain	0.1	0.0
Unrealized Loss	(0.1)	0.0
Allowance for Credit Losses	0.0
Fair Value	$ 81.9	$ 48.8

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- 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 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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