Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 20,797.8	$ 17,791.1
Gross Unrealized Gains	54.9	84.3
Gross unrealized losses	159.9	78.0
Held to Maturity	20,692.8	17,797.4
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	111.0	90.0
Gross Unrealized Gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Held to Maturity	111.0	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	6,208.8	3.0
Gross Unrealized Gains	5.0	0.3
Gross unrealized losses	47.4	0.0
Held to Maturity	6,166.4	3.3
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,792.8	8,336.6
Gross Unrealized Gains	5.7	7.3
Gross unrealized losses	3.7	0.2
Held to Maturity	2,794.8	8,343.7
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	945.8	588.0
Gross Unrealized Gains	4.2	6.5
Gross unrealized losses	2.3	0.1
Held to Maturity	947.7	594.4
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,988.5	3,184.6
Gross Unrealized Gains	12.2	24.6
Gross unrealized losses	3.4	0.3
Held to Maturity	2,997.3	3,208.9
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	777.8	807.2
Gross Unrealized Gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Held to Maturity	777.8	807.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,804.9	3,648.0
Gross Unrealized Gains	21.4	43.5
Gross unrealized losses	41.9	0.9
Held to Maturity	5,784.4	3,690.6
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	694.3	677.0
Gross Unrealized Gains	1.7	0.9
Gross unrealized losses	0.1	0.0
Held to Maturity	695.9	677.9
Other
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	472.9	454.6
Gross Unrealized Gains	4.7	1.1
Gross unrealized losses	61.1	76.5
Held to Maturity	$ 416.5	$ 379.2