Securities (Credit Quality Indicators) (Details) $ in Millions	Sep. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 22,708.6	$ 26,221.7
Percent of Total	1	1
AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 16,931.7	$ 18,234.4
Percent of Total	0.75	0.70
AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 3,880.2	$ 4,257.7
Percent of Total	0.17	0.16
A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 984.1	$ 2,834.7
Percent of Total	0.04	0.11
BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 337.1	$ 334.0
Percent of Total	0.01	0.01
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 575.5	$ 560.9
Percent of Total	0.03	0.02
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 2,549.9	$ 2,563.9
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	980.8	954.7
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1,569.1	1,609.0
Obligations of States and Political Subdivisions | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Obligations of States and Political Subdivisions | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Obligations of States and Political Subdivisions | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.2
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	8,826.8	9,355.3
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	8,826.8	9,355.3
Government Sponsored Agency | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Government Sponsored Agency | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Government Sponsored Agency | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Government Sponsored Agency | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	2,361.3	4,789.1
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	456.4	813.3
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	816.2	1,179.6
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	752.7	2,463.3
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	336.0	332.9
Non-U.S. Government | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	491.1	646.1
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	2.1
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	290.4	302.6
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	200.7	341.4
Corporate Debt | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Corporate Debt | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	2,124.8	2,208.6
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	2,124.8	2,208.6
Covered Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Covered Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Covered Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Covered Bonds | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	542.6	585.1
Certificates of Deposit | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	513.2	545.9
Certificates of Deposit | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Certificates of Deposit | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Certificates of Deposit | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Certificates of Deposit | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	29.4	39.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	5,062.1	5,245.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	3,825.8	4,047.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1,204.5	1,166.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	30.7	30.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1.1	1.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	111.6	214.2
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	111.6	214.2
Other Asset-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other Asset-Backed | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other Asset-Backed | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other Asset-Backed | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Commercial Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	37.6	37.6
Commercial Mortgage-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	37.6	37.6
Commercial Mortgage-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Commercial Mortgage-Backed | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Commercial Mortgage-Backed | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Commercial Mortgage-Backed | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	600.8	576.3
Other | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	54.7	54.8
Other | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 546.1	$ 521.5