Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 2,039.6	$ 909.5
Commercial Paper
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	31.0	21.9
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	134.7
Money Market Funds
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	741.7	673.8
Short-term Debt Securities
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 1,266.9	$ 79.1