Securities (Credit Quality Indicators) (Details) $ in Millions	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	$ 17,136.4	$ 17,791.1
Percent of Total	1	1
AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	$ 7,583.9	$ 6,869.1
Percent of Total	0.44	0.39
AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	$ 1,941.9	$ 2,674.6
Percent of Total	0.11	0.15
A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	$ 6,237.6	$ 6,935.7
Percent of Total	0.36	0.39
BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	$ 272.8	$ 49.9
Percent of Total	0.02	0
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	$ 1,100.2	$ 1,261.8
Percent of Total	0.07	0.07
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	$ 100.0	$ 90.0
U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	100.0	90.0
U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
U.S. Government | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	1.0	2.1
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	1.0	1.0
Obligations of States and Political Subdivisions | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Obligations of States and Political Subdivisions | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	1.1
Obligations of States and Political Subdivisions | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	53.6	3.0
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	53.6	3.0
Government Sponsored Agency | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Government Sponsored Agency | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Government Sponsored Agency | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Government Sponsored Agency | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	6,539.3	8,336.6
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	301.0	319.8
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	205.1	1,337.4
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	5,760.4	6,630.6
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	272.8	48.8
Non-U.S. Government | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	916.7	588.0
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	75.1	3.8
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	396.0	279.1
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	445.6	305.1
Corporate Debt | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Corporate Debt | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	3,046.8	3,184.6
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	3,046.8	3,184.6
Covered Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Covered Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Covered Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Covered Bonds | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	644.1	807.2
Certificates of Deposit | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Certificates of Deposit | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Certificates of Deposit | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Certificates of Deposit | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Certificates of Deposit | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	644.1	807.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	4,672.6	3,648.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	3,301.2	2,590.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	1,339.8	1,057.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	31.6	0.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	706.2	677.0
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	706.2	677.0
Other Asset-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Other Asset-Backed | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Other Asset-Backed | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Other Asset-Backed | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Other
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	456.1	454.6
Other | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Other | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Other | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Other | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0	0.0
Other | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	$ 456.1	$ 454.6