Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 21,655.3	$ 17,791.1
Gross Unrealized Gains	70.1	84.3
Gross unrealized losses	145.8	78.0
Held to Maturity	21,579.6	17,797.4
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	99.0	90.0
Gross Unrealized Gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Held to Maturity	99.0	90.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1.0	2.1
Gross Unrealized Gains	0.0	0.1
Gross unrealized losses	0.0	0.0
Held to Maturity	1.0	2.2
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	6,499.2	3.0
Gross Unrealized Gains	7.6	0.3
Gross unrealized losses	23.9	0.0
Held to Maturity	6,482.9	3.3
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,802.2	8,336.6
Gross Unrealized Gains	6.7	7.3
Gross unrealized losses	37.4	0.2
Held to Maturity	3,771.5	8,343.7
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	880.9	588.0
Gross Unrealized Gains	4.8	6.5
Gross unrealized losses	1.0	0.1
Held to Maturity	884.7	594.4
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,933.1	3,184.6
Gross Unrealized Gains	16.0	24.6
Gross unrealized losses	1.6	0.3
Held to Maturity	2,947.5	3,208.9
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	906.2	807.2
Gross Unrealized Gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Held to Maturity	906.2	807.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,333.1	3,648.0
Gross Unrealized Gains	28.7	43.5
Gross unrealized losses	19.0	0.9
Held to Maturity	5,342.8	3,690.6
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	734.8	677.0
Gross Unrealized Gains	1.9	0.9
Gross unrealized losses	0.0	0.0
Held to Maturity	736.7	677.9
Other
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	465.8	454.6
Gross Unrealized Gains	4.4	1.1
Gross unrealized losses	62.9	76.5
Held to Maturity	$ 407.3	$ 379.2