Securities (Credit Quality Indicators) (Details) $ in Millions	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 23,564.5	$ 17,791.1
Percentage of debt securities, held-to-maturity, portfolio	1	1
AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 14,207.5	$ 6,869.1
Percentage of debt securities, held-to-maturity, portfolio	0.60	0.39
AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 3,188.1	$ 2,674.6
Percentage of debt securities, held-to-maturity, portfolio	0.14	0.15
A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 4,632.9	$ 6,935.7
Percentage of debt securities, held-to-maturity, portfolio	0.20	0.39
BBB
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 345.0	$ 49.9
Percentage of debt securities, held-to-maturity, portfolio	0.01	0
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,191.0	$ 1,261.8
Percentage of debt securities, held-to-maturity, portfolio	0.05	0.07
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 47.0	$ 90.0
U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	47.0	90.0
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	0.8	2.1
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0	0.0
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.8	1.0
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	1.1
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	5,927.6	3.0
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,927.6	3.0
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	5,773.3	8,336.6
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	398.0	319.8
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	942.6	1,337.4
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	4,088.8	6,630.6
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	343.9	48.8
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	901.8	588.0
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2.3	3.8
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	386.7	279.1
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	512.8	305.1
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	2,942.4	3,184.6
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,942.4	3,184.6
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	674.7	807.2
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	674.7	807.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	6,098.0	3,648.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	4,207.6	2,590.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,858.0	1,057.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	31.3	0.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1.1	0.0
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	682.6	677.0
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	682.6	677.0
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	516.3	454.6
Other | AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0	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	$ 516.3	$ 454.6