Financial Instruments (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, Estimated Fair Value	$ 620.2	$ 1,135.0
Due in one year or less, Amortized Cost	620.2	1,134.5
Due after one year through five years, Estimated Fair Value	573.1	1,744.3
Due after one year through five years, Amortized Cost	572.9	1,741.2
Due after five years, Estimated Fair Value	52.6	292.4
Due after five years, Amortized Cost	52.7	292.3
Total available-for-sale securities, Fair Value	1,245.9	3,171.7
Total available-for-sale securities, Amortized Cost	$ 1,245.8	$ 3,168.0