Investment Securities (Amortized Cost And Fair Value Of Debt Securities Available-For-Sale By Contractual Maturity Dates) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Total, Amortized Cost	$ 6,161.9
Total, Fair Value	$ 6,123.6
Total, Weighted Average Yield	2.54%
Mortgage-backed Securities, U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
After 5 but within 10 years, Amortized Cost	$ 198.7
Due after 10 years, Amortized Cost	4,811.5
Total, Amortized Cost	5,010.2
After 5 but within 10 years, Fair Value	196.9
Due after 10 years, Fair Value	4,753.3
Total, Fair Value	$ 4,950.2
After 5 but within 10 years, Weighted Average Yield	2.05%
Due after 10 years, Weighted Average Yield	2.50%
Total, Weighted Average Yield	2.48%
Mortgage-backed Securities, Non-agency securities
Schedule of Available-for-sale Securities [Line Items]
After 1 but within 5 years, Amortized Cost	$ 12.5
After 5 but within 10 years, Amortized Cost	6.9
Due after 10 years, Amortized Cost	277.9
Total, Amortized Cost	297.3
After 1 but within 5 years, Fair Value	12.5
After 5 but within 10 years, Fair Value	7.4
Due after 10 years, Fair Value	298.6
Total, Fair Value	$ 318.5
After 1 but within 5 years, Weighted Average Yield	5.16%
After 5 but within 10 years, Weighted Average Yield	4.56%
Due after 10 years, Weighted Average Yield	5.72%
Total, Weighted Average Yield	5.67%
U.S. government agency obligations
Schedule of Available-for-sale Securities [Line Items]
After 1 but within 5 years, Amortized Cost	$ 25.0
Total, Amortized Cost	25.0
After 1 but within 5 years, Fair Value	24.8
Total, Fair Value	$ 24.8
After 1 but within 5 years, Weighted Average Yield	2.14%
Total, Weighted Average Yield	2.14%
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Due within 1 year, Amortized Cost	$ 199.3
After 5 but within 10 years, Amortized Cost	98.4
Total, Amortized Cost	297.7
Due within 1 year, Fair Value	199.1
After 5 but within 10 years, Fair Value	98.6
Total, Fair Value	$ 297.7
Due within 1 year, Weighted Average Yield	1.19%
After 5 but within 10 years, Weighted Average Yield	2.44%
Total, Weighted Average Yield	1.60%
Supranational securities
Schedule of Available-for-sale Securities [Line Items]
Due within 1 year, Amortized Cost	$ 399.8
After 1 but within 5 years, Amortized Cost	50.0
Total, Amortized Cost	449.8
Due within 1 year, Fair Value	399.8
After 1 but within 5 years, Fair Value	49.7
Total, Fair Value	$ 449.5
Due within 1 year, Weighted Average Yield	1.19%
After 1 but within 5 years, Weighted Average Yield	2.02%
Total, Weighted Average Yield	1.28%
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Due within 1 year, Amortized Cost	$ 0.1
After 1 but within 5 years, Amortized Cost	0.1
After 5 but within 10 years, Amortized Cost	0.3
Due after 10 years, Amortized Cost	15.7
Total, Amortized Cost	16.2
Due within 1 year, Fair Value	0.1
After 1 but within 5 years, Fair Value	0.1
After 5 but within 10 years, Fair Value	0.3
Due after 10 years, Fair Value	15.3
Total, Fair Value	$ 15.8
Due within 1 year, Weighted Average Yield	2.36%
After 1 but within 5 years, Weighted Average Yield	2.56%
After 5 but within 10 years, Weighted Average Yield	2.70%
Due after 10 years, Weighted Average Yield	2.35%
Total, Weighted Average Yield	2.36%
Corporate — foreign
Schedule of Available-for-sale Securities [Line Items]
After 1 but within 5 years, Amortized Cost	$ 65.7
Total, Amortized Cost	65.7
After 1 but within 5 years, Fair Value	67.1
Total, Fair Value	$ 67.1
After 1 but within 5 years, Weighted Average Yield	6.12%
Total, Weighted Average Yield	6.12%