Financial Instruments (Details 2) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, Estimated Fair Value	$ 944.4	$ 448.1
Due in one year or less, Amortized Cost	944.1	447.8
Due after one year through five years, Estimated Fair Value	1,196.9	664.1
Due after one year through five years, Amortized Cost	1,196.3	662.4
Due after five years, Estimated Fair Value	152.4	79.0
Due after five years, Amortized Cost	152.7	78.8
Total available-for-sale securities, Fair Value	2,293.7	1,191.2
Total available-for-sale securities, Amortized Cost	$ 2,293.1	$ 1,189.0