Financial Instruments (Details) - USD ($) $ in Millions	Mar. 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	$ 820.6	$ 909.5
Commercial paper
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	7.4	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	90.2	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	488.7	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	$ 234.3	$ 79.1