Investment Securities (Amortized Cost and Fair Value of AFS and HTM Securities) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,810.6	$ 3,730.3
Gross Unrealized Gains	31.0	17.2
Gross Unrealized Losses	(42.1)	(39.3)
Fair Value	4,799.5	3,708.2
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,029.2	3,973.3
Gross Unrealized Gains	39.1	24.1
Gross Unrealized Losses	(45.2)	(43.1)
Fair Value	5,023.1	3,954.3
U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,124.7	2,073.6
Gross Unrealized Gains	2.3	1.6
Gross Unrealized Losses	(35.9)	(32.3)
Fair Value	3,091.1	2,042.9
Non-Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	429.0	471.7
Gross Unrealized Gains	28.7	15.6
Gross Unrealized Losses	(0.8)	(1.8)
Fair Value	456.9	485.5
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	549.9	649.9
Gross Unrealized Losses	(4.2)	(3.9)
Fair Value	545.7	646.0
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	372.3	299.9
Gross Unrealized Losses	(0.3)	(0.4)
Fair Value	372.0	299.5
Supranational Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	299.3	200.2
Fair Value	299.3	200.2
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,775.2	3,695.3
Gross Unrealized Gains	31.0	17.2
Gross Unrealized Losses	(41.2)	(38.4)
Fair Value	4,765.0	3,674.1
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	218.6	243.0
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	35.4	35.0
Gross Unrealized Losses	(0.9)	(0.9)
Fair Value	34.5	34.1
U.S. Government Agency Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	95.5	110.0
Gross Unrealized Gains	0.5	0.7
Gross Unrealized Losses	(2.7)	(3.3)
Fair Value	93.3	107.4
State And Municipal [Member]
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	18.8	27.7
Gross Unrealized Losses	(0.4)	(0.5)
Fair Value	18.4	27.2
Foreign Government [Member]
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value		2.4
Fair Value		2.4
Corporate - Foreign [Member]
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	104.3	102.9
Gross Unrealized Gains	7.6	6.2
Fair Value	111.9	109.1
Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	218.6	243.0
Gross Unrealized Gains	8.1	6.9
Gross Unrealized Losses	(3.1)	(3.8)
Fair Value	$ 223.6	$ 246.1