Marketable Securities - Schedule of Debt Securities Based on Contractual Maturity Dates (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Maturities within one year	$ 1,342.2	$ 593.8
Maturities after one year through five years	1,667.0	1,426.8
Total	$ 3,009.2	$ 2,020.6