Securities (Credit Quality Indicators) (Details) $ in Millions	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 22,296.7	$ 26,221.7
Percentage of debt securities, held-to-maturity, portfolio	1	1
AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 15,780.9	$ 18,234.4
Percentage of debt securities, held-to-maturity, portfolio	0.71	0.70
AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 3,404.1	$ 4,257.7
Percentage of debt securities, held-to-maturity, portfolio	0.15	0.16
A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 2,208.7	$ 2,834.7
Percentage of debt securities, held-to-maturity, portfolio	0.10	0.11
BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 312.6	$ 334.0
Percentage of debt securities, held-to-maturity, portfolio	0.01	0.01
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 590.4	$ 560.9
Percentage of debt securities, held-to-maturity, portfolio	0.03	0.02
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 2,548.2	$ 2,563.9
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	1,024.3	954.7
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	1,523.9	1,609.0
Obligations of States and Political Subdivisions | A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Obligations of States and Political Subdivisions | BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Obligations of States and Political Subdivisions | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.2
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	8,635.0	9,355.3
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	8,635.0	9,355.3
Government Sponsored Agency | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Government Sponsored Agency | A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Government Sponsored Agency | BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Government Sponsored Agency | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	3,735.8	4,789.1
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	700.0	813.3
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	704.2	1,179.6
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	2,020.1	2,463.3
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	311.5	332.9
Non-U.S. Government | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	351.6	646.1
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	2.1
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	191.5	302.6
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	160.1	341.4
Corporate Debt | BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Corporate Debt | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	1,776.8	2,208.6
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	1,776.8	2,208.6
Covered Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Covered Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Covered Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Covered Bonds | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	336.0	585.1
Certificates of Deposit | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	316.6	545.9
Certificates of Deposit | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Certificates of Deposit | A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Certificates of Deposit | BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Certificates of Deposit | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	19.4	39.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	4,146.9	5,245.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	3,132.8	4,047.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	984.5	1,166.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	28.5	30.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	1.1	1.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	107.1	214.2
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	107.1	214.2
Other Asset-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Other Asset-Backed | A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Other Asset-Backed | BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Other Asset-Backed | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Commercial Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	37.6	37.6
Commercial Mortgage-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	37.6	37.6
Commercial Mortgage-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Commercial Mortgage-Backed | A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Commercial Mortgage-Backed | BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Commercial Mortgage-Backed | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Other securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	621.7	576.3
Other securities [Member] | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	50.7	54.8
Other securities [Member] | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Other securities [Member] | A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Other securities [Member] | BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	0.0
Other securities [Member] | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 571.0	$ 521.5