Financial Instruments (Details 1) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 752.5	$ 716.3
Commercial paper
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	10.3	54.2
Overnight reverse repurchase agreements
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	308.6	305.0
Money market funds
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	427.2	321.2
Short-term debt securities
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 6.4	$ 35.9