Securities (Credit Quality Indicators) (Details) $ in Millions	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 26,221.7	$ 25,036.1
Percentage of debt securities, held-to-maturity, portfolio	1	1
AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 18,234.4	$ 18,179.9
Percentage of debt securities, held-to-maturity, portfolio	0.70	0.73
AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 4,257.7	$ 4,372.7
Percentage of debt securities, held-to-maturity, portfolio	0.16	0.17
A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,834.7	$ 1,657.4
Percentage of debt securities, held-to-maturity, portfolio	0.11	0.07
BBB
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 334.0	$ 323.4
Percentage of debt securities, held-to-maturity, portfolio	0.01	0.01
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 560.9	$ 502.7
Percentage of debt securities, held-to-maturity, portfolio		0.02
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0	$ 50.0
U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0	50.0
U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0	0.0
U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0	0.0
U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0	0.0
U.S. Government | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0	0.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,563.9	2,565.3
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	954.7	926.8
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,609.0	1,638.5
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	0.2	0.0
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,355.3	9,407.7
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,355.3	9,407.7
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	4,789.1	3,234.0
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	813.3	762.2
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,179.6	926.5
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,463.3	1,223.0
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	332.9	322.3
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	646.1	713.3
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2.1	2.1
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	302.6	305.7
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	341.4	405.5
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	2,208.6	2,530.3
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,208.6	2,530.3
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	585.1	35.9
Certificates of Deposit | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	545.9	0.0
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	39.2	35.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,245.5	5,703.3
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	4,047.9	4,171.3
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,166.5	1,502.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	30.0	28.9
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	214.2	263.7
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	214.2	263.7
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
Commercial Mortgage-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	37.6
Commercial Mortgage-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0
Commercial Mortgage-Backed | A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0
Commercial Mortgage-Backed | BBB
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0
Commercial Mortgage-Backed | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0
Other
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	576.3	532.6
Other | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	54.8	65.8
Other | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0	0.0
Other | A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0	0.0
Other | BBB
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0	0.0
Other | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 521.5	$ 466.8