Financial Instruments (Details 2) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, Estimated Fair Value	$ 431.6	$ 620.2
Due in one year or less, Amortized Cost	431.5	620.2
Due after one year through five years, Estimated Fair Value	662.2	573.1
Due after one year through five years, Amortized Cost	662.0	572.9
Due after five years, Estimated Fair Value	62.0	52.6
Due after five years, Amortized Cost	62.1	52.7
Total available-for-sale securities, Fair Value	1,155.8	1,245.9
Total available-for-sale securities, Amortized Cost	$ 1,155.6	$ 1,245.8