Fair Value Measurements (Recurring Basis Hierarchy Leveling) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	$ 25,638.9	$ 23,089.8
Derivative Assets	1,761.1	3,568.2
NETTING	(914.2)	(3,126.7)
Derivative Assets	846.9	441.5
Derivative Liabilities	1,817.0	3,649.8
NETTING	(1,070.1)	(2,205.4)
Derivative Liabilities	746.9	1,444.4
Cross Product Collateral Adjustment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(181.0)	(2,093.8)
NETTING	(336.9)	(1,172.5)
U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	6,112.3	3,622.2
Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	295.0	295.8
Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	12,351.4	11,553.0
Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	307.6	264.4
Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	250.2	279.5
Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	296.8	347.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,860.8	2,899.9
Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,523.0	2,962.6
Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	641.8	865.3
LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	19,500.0	19,500.0
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	25,638.9	23,089.8
Trading Account		0.0
Total Available for Sale Debt Securities		23,089.8
Fair Value, Measurements, Recurring | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	6,112.3	3,622.2
Fair Value, Measurements, Recurring | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	295.0	295.8
Fair Value, Measurements, Recurring | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	12,351.4	11,553.0
Fair Value, Measurements, Recurring | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	307.6	264.4
Fair Value, Measurements, Recurring | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	250.2	279.5
Fair Value, Measurements, Recurring | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	296.8	347.1
Fair Value, Measurements, Recurring | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,860.8	2,899.9
Fair Value, Measurements, Recurring | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,523.0	2,962.6
Fair Value, Measurements, Recurring | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	641.8	865.3
Fair Value, Measurements, Recurring | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Investment	95.0	95.0
NETTING	(914.2)	(3,126.7)
Derivative Assets	846.9	441.5
Fair Value, Measurements, Recurring | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(699.0)	(2,937.2)
Derivative Assets	658.1	329.5
Fair Value, Measurements, Recurring | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(215.2)	(189.5)
Derivative Assets	188.8	112.0
Fair Value, Measurements, Recurring | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(1,070.1)	(2,205.4)
Derivative Liabilities	746.9	1,444.4
Fair Value, Measurements, Recurring | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(1,051.2)	(2,175.7)
Derivative Liabilities	266.9	1,079.5
Fair Value, Measurements, Recurring | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(2.7)	(6.0)
Derivative Liabilities	475.2	363.2
Fair Value, Measurements, Recurring | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(16.2)	(23.7)
Derivative Liabilities	4.8	1.7
Fair Value, Measurements, Recurring | LEVEL 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	6,112.3	3,622.2
Trading Account		0.0
Total Available for Sale Debt Securities		3,622.2
Fair Value, Measurements, Recurring | LEVEL 1 | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	6,112.3	3,622.2
Fair Value, Measurements, Recurring | LEVEL 1 | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Investment	95.0	95.0
Derivative Assets	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	19,526.6	19,467.6
Trading Account		0.0
Total Available for Sale Debt Securities		19,467.6
Fair Value, Measurements, Recurring | LEVEL 2 | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 2 | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	295.0	295.8
Fair Value, Measurements, Recurring | LEVEL 2 | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	12,351.4	11,553.0
Fair Value, Measurements, Recurring | LEVEL 2 | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	307.6	264.4
Fair Value, Measurements, Recurring | LEVEL 2 | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	250.2	279.5
Fair Value, Measurements, Recurring | LEVEL 2 | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	296.8	347.1
Fair Value, Measurements, Recurring | LEVEL 2 | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,860.8	2,899.9
Fair Value, Measurements, Recurring | LEVEL 2 | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,523.0	2,962.6
Fair Value, Measurements, Recurring | LEVEL 2 | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	641.8	865.3
Fair Value, Measurements, Recurring | LEVEL 2 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Investment	0.0	0.0
Derivative Assets	1,761.1	3,568.2
Fair Value, Measurements, Recurring | LEVEL 2 | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	1,357.1	3,266.7
Fair Value, Measurements, Recurring | LEVEL 2 | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	404.0	301.5
Fair Value, Measurements, Recurring | LEVEL 2 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	1,796.0	3,624.4
Fair Value, Measurements, Recurring | LEVEL 2 | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	1,318.1	3,255.2
Fair Value, Measurements, Recurring | LEVEL 2 | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	477.9	369.2
Fair Value, Measurements, Recurring | LEVEL 2 | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Trading Account		0.0
Total Available for Sale Debt Securities		0.0
Fair Value, Measurements, Recurring | LEVEL 3 | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Investment	0.0	0.0
Derivative Assets	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	21.0	25.4
Fair Value, Measurements, Recurring | LEVEL 3 | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	$ 21.0	$ 25.4