Corporate Investments and Funds Held For Clients (Expected Maturities Of Available-For-Sale Securities) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Corporate Investments And Funds Held For Clients [Abstract]
One year or less	$ 2,288.5
One year to two years	4,112.2
Two years to three years	5,290.8
Three years to four years	4,556.3
After four years	6,127.7
Total available-for-sale securities	$ 22,375.5