Securities (Credit Quality Indicators) (Details) $ in Millions	Jun. 30, 2026 USD ($)	Dec. 31, 2025 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 23,492.1	$ 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	$ 7,943.2	$ 7,015.3
Percentage of debt securities, held-to-maturity, portfolio	0.34	0.30
AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 11,823.5	$ 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	$ 2,351.3	$ 3,245.5
Percentage of debt securities, held-to-maturity, portfolio	0.10	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	$ 1,358.0	$ 1,060.7
Percentage of debt securities, held-to-maturity, portfolio	0.06	0.04
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 2,384.8	$ 2,457.8
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	993.0	986.0
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1,377.1	1,471.8
Obligations of States and Political Subdivisions | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	14.7	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	8,112.8	8,424.5
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	41.6	0.0
Government Sponsored Agency | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	8,071.2	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	4,561.2	4,741.0
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1,275.7	649.7
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1,457.5	1,231.7
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1,813.1	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	280.3	389.0
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	154.6	159.2
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	16.0	150.2
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	109.7	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	2,114.9	1,754.5
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	2,114.9	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	751.8	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	751.8	444.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	4,595.4	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	3,316.3	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	901.7	776.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	376.2	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
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	47.1
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	47.1
Other Asset-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0
Other Asset-Backed | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0
Other Asset-Backed | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0
Other Asset-Backed | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	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	0.0	37.6
Commercial Mortgage-Backed | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	37.6	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	606.2	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	$ 606.2	$ 616.2