Securities (Credit Quality Indicators) (Details) $ in Millions	Sep. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 24,920.4	$ 25,036.1
Percent of Total	1	1
AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 17,961.3	$ 18,179.9
Percent of Total	0.72	0.73
AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 4,089.2	$ 4,372.7
Percent of Total	0.17	0.17
A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 1,986.2	$ 1,657.4
Percent of Total	0.08	0.07
BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 320.1	$ 323.4
Percent of Total	0.01	0.01
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 563.6	$ 502.7
Percent of Total	0.02	0.02
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		$ 50.0
U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		50.0
U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.0
U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.0
U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.0
U.S. Government | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 2,577.7	2,565.3
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	918.0	926.8
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1,659.7	1,638.5
Obligations of States and Political Subdivisions | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Obligations of States and Political Subdivisions | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Obligations of States and Political Subdivisions | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	9,159.9	9,407.7
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	9,159.9	9,407.7
Government Sponsored Agency | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Government Sponsored Agency | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Government Sponsored Agency | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Government Sponsored Agency | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	3,711.8	3,234.0
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	894.1	762.2
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	879.0	926.5
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1,619.7	1,223.0
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	319.0	322.3
Non-U.S. Government | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	639.1	713.3
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	2.2	2.1
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	299.2	305.7
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	337.7	405.5
Corporate Debt | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Corporate Debt | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	2,116.9	2,530.3
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	2,116.9	2,530.3
Covered Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Covered Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Covered Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Covered Bonds | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	405.4	35.9
Certificates of Deposit | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	367.4	0.0
Certificates of Deposit | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Certificates of Deposit | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Certificates of Deposit | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Certificates of Deposit | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	38.0	35.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	5,443.8	5,703.3
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	4,162.6	4,171.3
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1,251.3	1,502.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	28.8	28.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1.1	1.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	246.6	263.7
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	246.6	263.7
Other Asset-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other Asset-Backed | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other Asset-Backed | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other Asset-Backed | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Commercial Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	37.6
Commercial Mortgage-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	37.6
Commercial Mortgage-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Commercial Mortgage-Backed | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Commercial Mortgage-Backed | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Commercial Mortgage-Backed | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Other
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	581.6	532.6
Other | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	56.0	65.8
Other | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Other | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 525.6	$ 466.8