Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 3,773.7	$ 1,229.4
Commercial paper
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	51.5	30.5
Overnight reverse repurchase agreements
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	0.0	3.6
Money market funds
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	3,561.2	948.0
Short-term debt securities
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 161.0	$ 247.3