Investment Securities (Schedule of Amortized Cost and Fair Value Maturities with Changes Recorded in Net Income) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total debt securities available-for-sale, Amortized Cost	$ 4,775.2
Total debt securities available-for-sale, Fair Value	$ 4,765.0
Weighted Average Yield	2.35%
U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
After 5 but within 10 years, Amortized Cost	$ 49.2
Due after 10 years, Amortized Cost	3,075.5
Total debt securities available-for-sale, Amortized Cost	3,124.7
After 5 but within 10 years, Fair Value	48.4
Due after 10 years, Fair Value	3,042.7
Total debt securities available-for-sale, Fair Value	$ 3,091.1
After 5 but within 10 years, Weighted Average Yield	1.52%
Due after 10 years, Weighted Average Yield	2.36%
Weighted Average Yield	2.35%
Non-Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
After 5 but within 10 years, Amortized Cost	$ 21.5
Due after 10 years, Amortized Cost	407.5
Total debt securities available-for-sale, Amortized Cost	429.0
After 5 but within 10 years, Fair Value	21.5
Due after 10 years, Fair Value	435.4
Total debt securities available-for-sale, Fair Value	$ 456.9
After 5 but within 10 years, Weighted Average Yield	4.94%
Due after 10 years, Weighted Average Yield	6.01%
Weighted Average Yield	5.95%
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
After 1 but within 5 years, Amortized Cost	$ 549.9
Total debt securities available-for-sale, Amortized Cost	549.9
After 1 but within 5 years, Fair Value	545.7
Total debt securities available-for-sale, Fair Value	$ 545.7
After 1 but within 5 years, Weighted Average Yield	1.22%
Weighted Average Yield	1.22%
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Due within 1 year, Amortized Cost	$ 349.4
After 1 but within 5 years, Amortized Cost	22.9
Total debt securities available-for-sale, Amortized Cost	372.3
Due within 1 year, Fair Value	349.2
After 1 but within 5 years, Fair Value	22.8
Total debt securities available-for-sale, Fair Value	$ 372.0
Due within 1 year, Weighted Average Yield	0.88%
After 1 but within 5 years, Weighted Average Yield	1.01%
Weighted Average Yield	0.89%
Supranational Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Due within 1 year, Amortized Cost	$ 299.3
Total debt securities available-for-sale, Amortized Cost	299.3
Due within 1 year, Fair Value	299.3
Total debt securities available-for-sale, Fair Value	$ 299.3
Due within 1 year, Weighted Average Yield	1.06%
Weighted Average Yield	1.06%
State And Municipal [Member]
Schedule of Held-to-maturity Securities [Line Items]
Due within 1 year, Amortized Cost	$ 0.4
After 1 but within 5 years, Amortized Cost	0.3
After 5 but within 10 years, Amortized Cost	0.3
Due after 10 years, Amortized Cost	17.8
Carrying Value	18.8	$ 27.7
Due within 1 year, Fair Value	0.4
After 1 but within 5 years, Fair Value	0.3
After 5 but within 10 years, Fair Value	0.3
Due after 10 years, Fair Value	17.4
Fair Value	$ 18.4	27.2
Due within 1 year, Average Yield	2.09%
After 1 but within 5 years, Average Yield	2.46%
After 5 but within 10 years, Average Yield	2.70%
Due after 10 years, Average Yield	2.33%
Average Yield	2.34%
U.S. Government Agency Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Due after 10 years, Amortized Cost	$ 95.5
Carrying Value	95.5	110.0
Due after 10 years, Fair Value	93.3
Fair Value	$ 93.3	107.4
Due after 10 years, Average Yield	2.42%
Average Yield	2.42%
Corporate - Foreign [Member]
Schedule of Held-to-maturity Securities [Line Items]
After 1 but within 5 years, Amortized Cost	$ 104.3
Carrying Value	104.3	102.9
After 1 but within 5 years, Fair Value	111.9
Fair Value	$ 111.9	109.1
After 1 but within 5 years, Average Yield	4.16%
Average Yield	4.16%
Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	$ 218.6	243.0
Fair Value	$ 223.6	$ 246.1
Average Yield	3.24%