Financial Instruments (Details 1) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 716.3	$ 424.7
Commercial Paper
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	54.2	1.2
Overnight Reverse Repurchase Agreements
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	305.0	22.4
Money Market Funds
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	321.2	396.0
Short-term Debt Securities
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 35.9	$ 5.1