Securities (Credit Quality Indicators) (Details) $ in Millions	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 23,497.9	$ 26,221.7
Percent of Total	1	1
AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 17,669.4	$ 18,234.4
Percent of Total	0.75	0.70
AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 3,773.9	$ 4,257.7
Percent of Total	0.16	0.16
A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 1,151.8	$ 2,834.7
Percent of Total	0.05	0.11
BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 326.2	$ 334.0
Percent of Total	0.01	0.01
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 576.6	$ 560.9
Percent of Total	0.03	0.02
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		$ 0.0
U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.0
U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.0
U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.0
U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.0
U.S. Government | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 2,571.6	2,563.9
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	956.9	954.7
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1,614.7	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	9,223.3	9,355.3
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	9,223.3	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	2,742.1	4,789.1
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	896.7	813.3
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	707.4	1,179.6
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	812.9	2,463.3
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	325.1	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	610.3	646.1
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	2.2	2.1
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	298.7	302.6
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	309.4	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	2,110.3	2,208.6
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	2,110.3	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	403.2	585.1
Certificates of Deposit | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	355.4	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	47.8	39.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	5,024.5	5,245.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	3,840.8	4,047.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1,153.1	1,166.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	29.5	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	193.7	214.2
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	193.7	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	581.3	576.3
Other | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	52.5	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	$ 528.8	$ 521.5