Marketable Securities - Schedule of Investments in Available For Sale Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 4,478.0	$ 4,206.3
Unrealized Gains	23.5	28.5
Unrealized Losses	(31.9)	(0.3)
Fair Value	4,469.6	4,234.5
Corporate bonds \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	4,128.3	3,960.5
Unrealized Gains	20.7	27.8
Unrealized Losses	(31.6)	(0.2)
Fair Value	4,117.4	3,988.1
U.S. government and government agency obligations \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	73.7	54.3
Unrealized Gains	1.4	0.2
Unrealized Losses	0.0	(0.1)
Fair Value	75.1	54.4
Sovereign bonds \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	36.0	26.9
Unrealized Gains	1.2	0.4
Unrealized Losses	0.0	0.0
Fair Value	37.2	27.3
Commercial paper \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	162.6	92.3
Unrealized Gains	0.1	0.0
Unrealized Losses	0.0	0.0
Fair Value	162.7	92.3
Certificates of deposit \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	77.4	72.3
Unrealized Gains	0.1	0.1
Unrealized Losses	(0.3)	0.0
Fair Value	$ 77.2	$ 72.4

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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