Securities (Credit Quality Indicators) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	$ 16,754.0	$ 12,284.5
AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	6,872.7
AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	2,491.9
A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	6,172.5
BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	48.0
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	1,168.9
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	99.0	138.8
U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	99.0
U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
U.S. Government | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	2.6	10.1
Obligations of States and Political Subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Obligations of States and Political Subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.6
Obligations of States and Political Subdivisions | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.7
Obligations of States and Political Subdivisions | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	1.3
Obligations of States and Political Subdivisions | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	3.4	4.1
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	3.4
Government Sponsored Agency | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Government Sponsored Agency | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Government Sponsored Agency | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Government Sponsored Agency | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	7,551.8	4,076.0
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	335.6
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	1,269.7
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	5,899.8
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	46.7
Non-U.S. Government | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	530.3	405.1
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	3.6
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	254.7
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	272.0
Corporate Debt | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Corporate Debt | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	3,361.7	3,006.7
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	3,361.7
Covered Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Covered Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Covered Bonds | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Covered Bonds | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	726.3	262.9
Certificates of Deposit | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Certificates of Deposit | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Certificates of Deposit | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Certificates of Deposit | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Certificates of Deposit | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	726.3
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	3,390.5	3,285.4
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	2,423.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	966.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	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
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
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	645.8	804.3
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	645.8
Other Asset-Backed | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Other Asset-Backed | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Other Asset-Backed | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Other Asset-Backed | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Other
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	442.6	$ 291.1
Other | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Other | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Other | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Other | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	0.0
Other | NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	$ 442.6