Investment Securities (Schedule of Securities Carried at Fair Value with Changes Recorded in Net Income) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 228.6	$ 277.5
Gross Unrealized Gains	19.1	6.7
Gross Unrealized Losses	0.0	(0.7)
Fair Value	247.7	283.5
Mortgage-backed Securities — Non-agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	228.6	277.5
Gross Unrealized Gains	19.1	6.7
Gross Unrealized Losses	0.0	(0.7)
Fair Value	$ 247.7	$ 283.5