Securities (Credit Quality Indicators) (Details) $ in Millions	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 21,223.5	$ 23,564.5
Percent of Total	1	1
AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 14,530.5	$ 14,207.5
Percent of Total	0.68	0.60
AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 3,307.8	$ 3,188.1
Percent of Total	0.16	0.14
A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 1,686.5	$ 4,632.9
Percent of Total	0.08	0.20
BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 337.4	$ 345.0
Percent of Total	0.02	0.01
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 1,361.3	$ 1,191.0
Percent of Total	0.06	0.05
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 49.0	$ 47.0
U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	49.0	47.0
U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
U.S. Government | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.8
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.0
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.8
Obligations of States and Political Subdivisions | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.0
Obligations of States and Political Subdivisions | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.0
Obligations of States and Political Subdivisions | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity		0.0
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	6,079.8	5,927.6
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	6,079.8	5,927.6
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	2,950.9	5,773.3
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	501.3	398.0
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1,036.3	942.6
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1,077.0	4,088.8
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	336.3	343.9
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	932.2	901.8
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	2.3	2.3
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	382.5	386.7
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	547.4	512.8
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	2,941.9	2,942.4
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	2,941.9	2,942.4
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	892.1	674.7
Certificates of Deposit | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	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	892.1	674.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	6,223.6	6,098.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	4,271.4	4,207.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1,889.0	1,858.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	62.1	31.3
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	608.4	682.6
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	608.4	682.6
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
Other
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	545.6	516.3
Other | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	76.4	0.0
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	$ 469.2	$ 516.3