Corporate Investments and Funds Held For Clients (Expected Maturities Of Available-For-Sale Securities) (Details) $ in Millions	Sep. 30, 2017 USD ($)
Corporate Investments And Funds Held For Clients [Abstract]
One year or less	$ 2,430.3
One year to two years	3,878.9
Two years to three years	4,835.9
Three years to four years	5,108.2
After four years	5,853.3
Total available-for-sale securities	$ 22,106.6