Financial Instruments (Details 2) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, estimated fair value	$ 2,434.8	$ 2,120.5
Due in one year or less, amortized cost	2,434.1	2,122.0
Due after one year through five years, Estimated Fair Value	3,255.9	2,575.9
Due after one year through five years, Amortized Cost	3,245.4	2,583.9
Due after five years, estimated fair value	221.7	184.5
Due after five years, amortized cost	221.8	185.3
Total available-for-sale securities, Fair Value	5,912.4	4,880.9
Total available-for-sale securities, Amortized Cost	$ 5,901.3	$ 4,891.2