Marketable Securities - Schedule of Investments in Available For Sale Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 3,730.2	$ 3,026.6
Unrealized Gains	9.7	1.0
Unrealized Losses	(6.9)	(18.4)
Available-for-sale debt securities:	3,733.0	3,009.2
Corporate bonds \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	3,352.4	2,734.8
Unrealized Gains	9.3	1.0
Unrealized Losses	(6.3)	(17.4)
Available-for-sale debt securities:	3,355.4	2,718.4
U.S. government and government agency obligations \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	129.7	110.4
Unrealized Gains	0.1	0.0
Unrealized Losses	(0.6)	(1.0)
Available-for-sale debt securities:	129.2	109.4
Sovereign bonds \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	26.8	7.6
Unrealized Gains	0.2	0.0
Unrealized Losses	0.0	0.0
Available-for-sale debt securities:	27.0	7.6
Commercial paper \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	152.0	113.8
Unrealized Gains	0.0	0.0
Unrealized Losses	0.0	0.0
Available-for-sale debt securities:	152.0	113.8
Certificates of deposit \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	69.3	60.0
Unrealized Gains	0.1	0.0
Unrealized Losses	0.0	0.0
Available-for-sale debt securities:	$ 69.4	$ 60.0

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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-Section 50
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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).

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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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