Financial Instruments (Details 2) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, estimated fair value	$ 1,956.2	$ 2,568.6
Due in one year or less, amortized cost	1,957.1	2,569.1
Due after one year through five years, Estimated Fair Value	2,561.9	2,552.6
Due after one year through five years, Amortized Cost	2,565.4	2,559.7
Due after five years, estimated fair value	263.3	276.8
Due after five years, amortized cost	263.5	277.5
Total available-for-sale securities, Fair Value	4,781.4	5,398.0
Total available-for-sale securities, Amortized Cost	$ 4,786.0	$ 5,406.3