Financial Instruments (Details 2) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, Estimated Fair Value	$ 275.2	$ 1,135.0
Due in one year or less, Amortized Cost	275.3	1,134.5
Due after one year through five years, Estimated Fair Value	103.9	1,744.3
Due after one year through five years, Amortized Cost	104.0	1,741.2
Due after five years, Estimated Fair Value	3.2	292.4
Due after five years, Amortized Cost	3.2	292.3
Total available-for-sale securities, Fair Value	382.3	3,171.7
Total available-for-sale securities, Amortized Cost	$ 382.5	$ 3,168.0