Financial Instruments (Details 2) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, estimated fair value	$ 673.9	$ 640.5
Due in one year or less, amortized cost	674.0	640.8
Due after one year through five years, Estimated Fair Value	1,252.0	1,343.7
Due after one year through five years, Amortized Cost	1,251.0	1,345.2
Due after five years, estimated fair value	125.3	126.9
Due after five years, amortized cost	125.1	126.8
Total available-for-sale securities, Fair Value	2,051.2	2,111.1
Total available-for-sale securities, Amortized Cost	$ 2,050.1	$ 2,112.8