Financial Instruments (Details 1) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 358.6	$ 439.4
Commercial paper [Member]
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	0	40.7
Overnight Repurchase Agreements [Member]
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	0	67.4
Short-term Debt Securities [Member]
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 358.6	$ 331.3