Investment Securities (Amortized Cost and Fair Value of Debt Securities Held-To-Maturity by Contractual Maturity Dates) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 243.0	$ 277.5
Fair Value	255.6	$ 283.5
Non-Agency Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost, After 5 but within 10 years	0.3
Fair Value, After 5 but within 10 years	$ 0.3
Weighted Average Yield, After 5 but within 10 years	41.82%
Amortized Cost, Due after 10 years	$ 242.7
Fair Value, Due after 10 years	$ 255.3
Weighted Average Yield, Due after 10 years	4.89%
Amortized Cost	$ 243.0
Fair Value	$ 255.6
Weighted Average Yield	4.94%