Securities (Credit Quality Indicators) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	$ 15,241.9	$ 12,284.5
AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	7,030.1
AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	2,319.9
A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	4,250.3
BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	46.0
NOT RATED
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	1,595.6
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	98.0	138.8
U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	98.0
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	7.0	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	0.5
Obligations of States and Political Subdivisions | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	5.2
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	3.6	4.1
Government Sponsored Agency | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	3.6
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	6,134.6	4,076.0
Non-U.S. Government | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	891.7
Non-U.S. Government | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	1,217.0
Non-U.S. Government | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	3,981.2
Non-U.S. Government | BBB
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	44.7
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	509.6	405.1
Corporate Debt | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	3.5
Corporate Debt | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	242.2
Corporate Debt | A
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	263.9
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	3,198.0	3,006.7
Covered Bonds | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	3,198.0
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	1,176.3	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	1,176.3
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	2,999.3	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	2,139.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | AA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	860.2
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	696.2	804.3
Other Asset-Backed | AAA
Schedule of Held-to-maturity Securities [Line Items]
Debt securities, held-to-maturity	696.2
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	419.3	$ 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	$ 419.3