Securities (Credit Quality Indicators) (Details) $ in Millions	Mar. 31, 2026 USD ($)	Dec. 31, 2025 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 24,286.2	$ 23,429.6
Percentage of debt securities, held-to-maturity, portfolio	1	1
AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 7,952.6	$ 7,015.3
Percentage of debt securities, held-to-maturity, portfolio	0.33	0.30
AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 12,082.1	$ 12,092.0
Percentage of debt securities, held-to-maturity, portfolio	0.50	0.52
A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 2,856.5	$ 3,245.5
Percentage of debt securities, held-to-maturity, portfolio	0.12	0.14
BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 16.1	$ 16.1
Percentage of debt securities, held-to-maturity, portfolio	0	0
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 1,378.9	$ 1,060.7
Percentage of debt securities, held-to-maturity, portfolio	0.05	0.04
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 2,442.1	$ 2,457.8
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1,000.7	986.0
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1,441.4	1,471.8
Obligations of States and Political Subdivisions | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Obligations of States and Political Subdivisions | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Obligations of States and Political Subdivisions | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	8,299.6	8,424.5
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	45.7	0.0
Government Sponsored Agency | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	8,253.9	8,424.5
Government Sponsored Agency | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Government Sponsored Agency | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Government Sponsored Agency | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	5,026.8	4,741.0
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1,258.5	649.7
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1,327.3	1,231.7
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	2,426.1	2,844.7
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	14.9	14.9
Non-U.S. Government | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	303.5	389.0
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	155.9	159.2
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	94.5	150.2
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	53.1	79.6
Corporate Debt | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Corporate Debt | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	2,192.8	1,754.5
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	2,192.8	1,754.5
Covered Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Covered Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Covered Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Covered Bonds | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Certificate of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	766.5	444.5
Certificate of Deposit | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Certificate of Deposit | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Certificate of Deposit | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Certificate of Deposit | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Certificate of Deposit | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	766.5	444.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	4,604.9	4,511.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	3,299.0	3,412.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	927.4	776.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	377.3	321.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1.2	1.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Commercial Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	37.6	37.6
Commercial Mortgage-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Commercial Mortgage-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	37.6	37.6
Commercial Mortgage-Backed | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Commercial Mortgage-Backed | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Commercial Mortgage-Backed | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Other
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	612.4	669.2
Other | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	53.0
Other | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Other | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Other | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Other | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 612.4	$ 616.2