Investment Securities (Schedule Of Interest And Dividend Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Dividends — investments	$ 10.9	$ 16.7	$ 10.4
Total interest and dividends	197.5	131.9	71.2
Investments / Reverse Repos
Net Investment Income [Line Items]
Interest income	128.9	82.1	43.7
Interest-bearing Deposits
Net Investment Income [Line Items]
Interest income	$ 57.7	$ 33.1	$ 17.1