Financial Instruments (Details 2) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, Estimated Fair Value	$ 2,115.2	$ 2,568.6
Due in one year or less, Amortized Cost	2,116.0	2,569.1
Due after one year through five years, Estimated Fair Value	2,730.0	2,552.6
Due after one year through five years, Amortized Cost	2,730.0	2,559.7
Due after five years, Estimated Fair Value	327.3	276.8
Due after five years, Amortized Cost	327.0	277.5
Total available-for-sale securities, Fair Value	5,172.5	5,398.0
Total available-for-sale securities, Amortized Cost	$ 5,173.0	$ 5,406.3