Marketable Securities - Schedule of Investments in Available For Sale Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 3,508.3	$ 4,206.3
Unrealized Gains	42.9	28.5
Unrealized Losses	(0.8)	(0.3)
Fair Value	3,550.4	4,234.5
Corporate bonds | Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	2,796.6	3,960.5
Unrealized Gains	40.1	27.8
Unrealized Losses	(0.7)	(0.2)
Fair Value	2,836.0	3,988.1
U.S. government and government agency obligations | Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	133.5	54.3
Unrealized Gains	1.3	0.2
Unrealized Losses	(0.1)	(0.1)
Fair Value	134.7	54.4
Sovereign bonds | Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	65.4	26.9
Unrealized Gains	1.2	0.4
Unrealized Losses	0.0	0.0
Fair Value	66.6	27.3
Commercial paper | Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	390.4	92.3
Unrealized Gains	0.2	0.0
Unrealized Losses	0.0	0.0
Fair Value	390.6	92.3
Certificates of deposit | Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	122.4	72.3
Unrealized Gains	0.1	0.1
Unrealized Losses	0.0	0.0
Fair Value	$ 122.5	$ 72.4