Securities (Credit Quality Indicators) (Details) $ in Millions	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 20,797.8	$ 17,791.1
Percent of Total	1	1
AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 14,713.9	$ 6,869.1
Percent of Total	0.71	0.39
AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 2,898.5	$ 2,674.6
Percent of Total	0.14	0.15
A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 1,600.4	$ 6,935.7
Percent of Total	0.08	0.39
BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 334.3	$ 49.9
Percent of Total	0.02	0
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 1,250.7	$ 1,261.8
Percent of Total	0.05	0.07
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	$ 111.0	$ 90.0
U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	111.0	90.0
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	1.0	2.1
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	0.0
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1.0	1.0
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	1.1
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	6,208.8	3.0
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	6,208.8	3.0
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	2,792.8	8,336.6
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	657.3	319.8
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	743.1	1,337.4
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1,059.3	6,630.6
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	333.1	48.8
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	945.8	588.0
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	49.2	3.8
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	387.2	279.1
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	509.4	305.1
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	2,988.5	3,184.6
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	2,988.5	3,184.6
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	777.8	807.2
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	777.8	807.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	5,804.9	3,648.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	4,004.8	2,590.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1,767.2	1,057.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	31.7	0.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	1.2	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	0.0
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	694.3	677.0
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	694.3	677.0
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	472.9	454.6
Other | AAA
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity	0.0	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	$ 472.9	$ 454.6