Financial Instruments (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, Estimated Fair Value	$ 640.5	$ 620.2
Due in one year or less, Amortized Cost	640.8	620.2
Due after one year through five years, Estimated Fair Value	1,343.7	573.1
Due after one year through five years, Amortized Cost	1,345.2	572.9
Due after five years, Estimated Fair Value	126.9	52.6
Due after five years, Amortized Cost	126.8	52.7
Total available-for-sale securities, Fair Value	2,111.1	1,245.9
Total available-for-sale securities, Amortized Cost	$ 2,112.8	$ 1,245.8