Securities (Credit Quality Indicators) (Details) $ in Millions	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 25,036.1	$ 23,564.5
Percentage of debt securities, held-to-maturity, portfolio	1	1
AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 18,179.9	$ 14,207.5
Percentage of debt securities, held-to-maturity, portfolio	0.73	0.60
AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 4,372.7	$ 3,188.1
Percentage of debt securities, held-to-maturity, portfolio	0.17	0.14
A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,657.4	$ 4,632.9
Percentage of debt securities, held-to-maturity, portfolio	0.07	0.20
BBB
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 323.4	$ 345.0
Percentage of debt securities, held-to-maturity, portfolio	0.01	0.01
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 502.7	$ 1,191.0
Percentage of debt securities, held-to-maturity, portfolio	0.02	0.05
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 50.0	$ 47.0
U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	50.0	47.0
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	2,565.3	0.8
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	926.8	0.0
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,638.5	0.8
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	9,407.7	5,927.6
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,407.7	5,927.6
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	3,234.0	5,773.3
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	762.2	398.0
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	926.5	942.6
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,223.0	4,088.8
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	322.3	343.9
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	713.3	901.8
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2.1	2.3
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	305.7	386.7
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	405.5	512.8
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	2,530.3	2,942.4
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,530.3	2,942.4
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	35.9	674.7
Certificates of Deposit | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0	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	35.9	674.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,703.3	6,098.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	4,171.3	4,207.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,502.0	1,858.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	28.9	31.3
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	263.7	682.6
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	263.7	682.6
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
Other
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	532.6	516.3
Other | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	65.8	0.0
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	$ 466.8	$ 516.3