Marketable Securities - Schedule of Investments in Available For Sale Debt Securities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 3,026.6	$ 2,027.3
Unrealized Gains	1.0	2.2
Unrealized Losses	(18.4)	(8.9)
Fair Value	3,009.2	2,020.6
Corporate bonds \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	2,734.8	1,718.0
Unrealized Gains	1.0	2.2
Unrealized Losses	(17.4)	(7.7)
Fair Value	2,718.4	1,712.5
U.S. government and government agency obligations \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	110.4	186.7
Unrealized Gains	0.0	0.0
Unrealized Losses	(1.0)	(1.2)
Fair Value	109.4	185.5
Sovereign bonds \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	7.6
Unrealized Gains	0.0
Unrealized Losses	0.0
Fair Value	7.6
Municipal and sovereign bonds \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis		4.6
Unrealized Gains		0.0
Unrealized Losses		0.0
Fair Value		4.6
Commercial paper \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	113.8	107.0
Unrealized Gains	0.0	0.0
Unrealized Losses	0.0	0.0
Fair Value	113.8	107.0
Certificates of deposit \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	60.0	11.0
Unrealized Gains	0.0	0.0
Unrealized Losses	0.0	0.0
Fair Value	$ 60.0	$ 11.0

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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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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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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