Securities (Credit Quality Indicators) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	$ 12,635.7	$ 12,284.5
U.S. Government
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	104.9	138.8
Obligations of States and Political Subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	6.9	10.1
Government Sponsored Agency
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	3.9	4.1
Non-U.S. Government
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	3,696.0	3,006.7
Corporate Debt
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	520.9	4,076.0
Covered Bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	3,116.9	405.1
Certificates of Deposit
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	1,041.9	262.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	3,060.0	3,285.4
Other Asset-Backed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	783.3	804.3
Other
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	301.0	$ 291.1
AAA
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	6,506.9
AAA | U.S. Government
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	104.9
AAA | Obligations of States and Political Subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
AAA | Government Sponsored Agency
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	3.9
AAA | Non-U.S. Government
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	348.1
AAA | Corporate Debt
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	3.4
AAA | Covered Bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	3,116.9
AAA | Certificates of Deposit
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
AAA | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	2,146.4
AAA | Other Asset-Backed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	783.3
AAA | Other
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
AA
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	2,358.8
AA | U.S. Government
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
AA | Obligations of States and Political Subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.5
AA | Government Sponsored Agency
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
AA | Non-U.S. Government
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	1,219.2
AA | Corporate Debt
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	256.5
AA | Covered Bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
AA | Certificates of Deposit
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
AA | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	882.6
AA | Other Asset-Backed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
AA | Other
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
A
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	2,394.8
A | U.S. Government
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
A | Obligations of States and Political Subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	5.1
A | Government Sponsored Agency
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
A | Non-U.S. Government
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	2,128.7
A | Corporate Debt
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	261.0
A | Covered Bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
A | Certificates of Deposit
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
A | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
A | Other Asset-Backed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
A | Other
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
BBB
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	1,373.9
BBB | U.S. Government
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
BBB | Obligations of States and Political Subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
BBB | Government Sponsored Agency
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
BBB | Non-U.S. Government
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
BBB | Corporate Debt
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
BBB | Covered Bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
BBB | Certificates of Deposit
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	1,041.9
BBB | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	31.0
BBB | Other Asset-Backed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
BBB | Other
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	301.0
NOT RATED
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	1.3
NOT RATED | U.S. Government
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
NOT RATED | Obligations of States and Political Subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	1.3
NOT RATED | Government Sponsored Agency
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
NOT RATED | Non-U.S. Government
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
NOT RATED | Corporate Debt
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
NOT RATED | Covered Bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
NOT RATED | Certificates of Deposit
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
NOT RATED | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
NOT RATED | Other Asset-Backed
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	0.0
NOT RATED | Other
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Held to Maturity	$ 0.0