Securities (Credit Quality Indicators) (Details) $ in Millions	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 17,791.1	$ 12,284.5
Percentage of debt securities, held-to-maturity, portfolio	1
AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 6,869.1
Percentage of debt securities, held-to-maturity, portfolio	0.39
AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 2,674.6
Percentage of debt securities, held-to-maturity, portfolio	0.15
A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 6,935.7
Percentage of debt securities, held-to-maturity, portfolio	0.39
BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 49.9
Percentage of debt securities, held-to-maturity, portfolio	0
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 1,261.8
Percentage of debt securities, held-to-maturity, portfolio	0.07
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 90.0	138.8
U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	90.0
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	2.1	10.1
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	1.0
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	1.1
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	3.0	4.1
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	3.0
Government Sponsored Agency | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Government Sponsored Agency | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Government Sponsored Agency | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Government Sponsored Agency | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	8,336.6	4,076.0
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	319.8
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1,337.4
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	6,630.6
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	48.8
Non-U.S. Government | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	588.0	405.1
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	3.8
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	279.1
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	305.1
Corporate Debt | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Corporate Debt | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	3,184.6	3,006.7
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	3,184.6
Covered Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Covered Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Covered Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Covered Bonds | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	807.2	262.9
Certificates of Deposit | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Certificates of Deposit | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Certificates of Deposit | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Certificates of Deposit | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Certificates of Deposit | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	807.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	3,648.0	3,285.4
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	2,590.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1,057.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	677.0	804.3
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	677.0
Other Asset-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Other Asset-Backed | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Other Asset-Backed | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Other Asset-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	454.6	$ 291.1
Other | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Other | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Other | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Other | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0
Other | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 454.6