Investment Securities (Schedule of Interest and Dividend Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net Investment Income [Line Items]
Dividends — investments	$ 2.5	$ 3.2	$ 8.3	$ 9.6
Total interest and dividends	50.5	31.9	151.0	93.9
Investments
Net Investment Income [Line Items]
Interest income	35.5	19.8	93.8	58.7
Interest-bearing Deposits
Net Investment Income [Line Items]
Interest income	$ 12.5	$ 8.9	$ 48.9	$ 25.6