Corporate Investments and Funds Held For Clients (Expected Maturities Of Available-For-Sale Securities) (Details) $ in Millions	Mar. 31, 2018 USD ($)
Corporate Investments And Funds Held For Clients [Abstract]
One year or less	$ 2,420.5
One year to two years	4,783.9
Two years to three years	5,690.6
Three years to four years	3,808.7
After four years	5,799.0
Total available-for-sale securities	$ 22,502.7