Securities (Credit Quality Indicators) (Details) $ in Millions	Sep. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 22,974.9	$ 22,296.7
Percentage of debt securities, held-to-maturity, portfolio	1	1
AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 6,920.6	$ 15,780.9
Percentage of debt securities, held-to-maturity, portfolio	0.30	0.71
AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 13,661.6	$ 3,404.1
Percentage of debt securities, held-to-maturity, portfolio	0.59	0.15
A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 1,416.5	$ 2,208.7
Percentage of debt securities, held-to-maturity, portfolio	0.06	0.10
BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 345.5	$ 312.6
Percentage of debt securities, held-to-maturity, portfolio	0.02	0.01
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 630.7	$ 590.4
Percentage of debt securities, held-to-maturity, portfolio	0.03	0.03
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 2,476.0	$ 2,548.2
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1,002.6	1,024.3
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1,473.4	1,523.9
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	8,512.4	8,635.0
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	8,635.0
Government Sponsored Agency | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	8,512.4	0.0
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	3,405.6	3,735.8
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	596.1	700.0
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1,196.5	704.2
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1,268.7	2,020.1
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	344.3	311.5
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	407.2	351.6
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	159.4	0.0
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	150.5	191.5
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	97.3	160.1
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	1,543.4	1,776.8
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1,290.2	1,776.8
Covered Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	253.2	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
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	912.0	336.0
Certificates of Deposit | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	316.6
Certificates of Deposit | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	861.3	0.0
Certificates of Deposit | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Certificates of Deposit | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Certificates of Deposit | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	50.7	19.4
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	5,047.4	4,146.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	3,819.0	3,132.8
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1,176.7	984.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	50.5	28.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	1.2	1.1
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		107.1
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities		107.1
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	37.6
Commercial Mortgage-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	37.6	0.0
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 securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	633.3	621.7
Other securities [Member] | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	53.3	50.7
Other securities [Member] | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Other securities [Member] | A
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Other securities [Member] | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	0.0	0.0
Other securities [Member] | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities	$ 580.0	$ 571.0