Securities (Credit Quality Indicators) (Details) $ in Millions	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 23,429.6	$ 22,296.7
Percentage of debt securities, held-to-maturity, portfolio	1	1
AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 7,015.3	$ 15,780.9
Percentage of debt securities, held-to-maturity, portfolio	0.30	0.71
AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 12,092.0	$ 3,404.1
Percentage of debt securities, held-to-maturity, portfolio	0.52	0.15
A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 3,245.5	$ 2,208.7
Percentage of debt securities, held-to-maturity, portfolio	0.14	0.10
BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 16.1	$ 312.6
Percentage of debt securities, held-to-maturity, portfolio	0	0.01
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 1,060.7	$ 590.4
Percentage of debt securities, held-to-maturity, portfolio	0.04	0.03
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 2,457.8	$ 2,548.2
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	986.0	1,024.3
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	1,471.8	1,523.9
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	0.0
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	8,424.5	8,635.0
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	8,635.0
Government Sponsored Agency | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	8,424.5	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	4,741.0	3,735.8
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	649.7	700.0
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	1,231.7	704.2
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	2,844.7	2,020.1
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	14.9	311.5
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	389.0	351.6
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	159.2	0.0
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	150.2	191.5
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	79.6	160.1
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	1,754.5	1,776.8
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	1,754.5	1,776.8
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	444.5	336.0
Certificates of Deposit | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	0.0	316.6
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	444.5	19.4
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	4,511.5	4,146.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	3,412.9	3,132.8
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	776.2	984.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	321.2	28.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	1.2	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
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST		107.1
Other Asset-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST		0.0
Other Asset-Backed | A
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST		0.0
Other Asset-Backed | BBB
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST		0.0
Other Asset-Backed | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST		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	0.0	37.6
Commercial Mortgage-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	37.6	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	669.2	621.7
Other securities [Member] | AAA
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	53.0	50.7
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	$ 616.2	$ 571.0