Investment Securities (Amortized Cost and Fair Value of AFS and HTM Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 5,013.2	$ 3,730.3
Gross Unrealized Gains	35.0	17.2
Gross Unrealized Losses	(39.9)	(39.3)
Fair Value	5,008.3	3,708.2
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,224.5	3,973.3
Gross Unrealized Gains	42.5	24.1
Gross Unrealized Losses	(42.6)	(43.1)
Fair Value	5,224.4	3,954.3
U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,499.9	2,073.6
Gross Unrealized Gains	4.2	1.6
Gross Unrealized Losses	(33.2)	(32.3)
Fair Value	3,470.9	2,042.9
Non-agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	381.5	471.7
Gross Unrealized Gains	30.7	15.6
Gross Unrealized Losses	(1.2)	(1.8)
Fair Value	411.0	485.5
U.S. government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	574.9	649.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(4.4)	(3.9)
Fair Value	570.5	646.0
U.S. Treasury Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	421.5	299.9
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(0.2)	(0.4)
Fair Value	421.4	299.5
Supranational securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	99.8	200.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	99.8	200.2
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,977.6	3,695.3
Gross Unrealized Gains	35.0	17.2
Gross Unrealized Losses	(39.0)	(38.4)
Fair Value	4,973.6	3,674.1
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	211.3	243.0
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	35.6	35.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.9)	(0.9)
Fair Value	34.7	34.1
U.S. government agency securities
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	89.2	110.0
Gross Unrealized Gains	0.4	0.7
Gross Unrealized Losses	(2.4)	(3.3)
Fair Value	87.2	107.4
State and municipal
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	16.3	27.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.3)	(0.5)
Fair Value	16.0	27.2
Foreign government
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value		2.4
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Fair Value		2.4
Corporate — foreign
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	105.8	102.9
Gross Unrealized Gains	7.1	6.2
Gross Unrealized Losses	0.0	0.0
Fair Value	112.9	109.1
Debt securities
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	211.3	243.0
Gross Unrealized Gains	7.5	6.9
Gross Unrealized Losses	(2.7)	(3.8)
Fair Value	$ 216.1	$ 246.1