Financial Instruments (Details 2) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, Estimated Fair Value	$ 240.9	$ 1,135.0
Due in one year or less, Amortized Cost	240.9	1,134.5
Due after one year through five years, Estimated Fair Value	351.8	1,744.3
Due after one year through five years, Amortized Cost	351.6	1,741.2
Due after five years, Estimated Fair Value	21.7	292.4
Due after five years, Amortized Cost	21.9	292.3
Total available-for-sale securities, Fair Value	614.4	3,171.7
Total available-for-sale securities, Amortized Cost	$ 614.4	$ 3,168.0