Fair Value Measurements - Recurring Basis Hierarchy Leveling (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	$ 29,001.5	$ 23,089.8
Derivative Assets, before Netting	5,358.5	3,568.2
Derivative Assets, Netting	(1,910.4)	(3,126.7)
Derivative Asset	3,448.1	441.5
Derivative Liabilities, before Netting	5,158.4	3,649.8
Liability, Netting	(4,199.6)	(2,205.4)
Derivative Liabilities	958.8	1,444.4
Money Market Investment	85.0
Equity securities	26.3
Cross Product Collateral Adjustment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets, Netting	(368.2)	(2,100.0)
Liability, Netting	(2,700.0)	(1,200.0)
LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	21,600.0	19,500.0
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	29,001.5	23,089.8
Equity Securities and Money Market Investment		95.0
Fair Value, Measurements, Recurring | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Equity Securities and Money Market Investment	111.3
Derivative Assets, Netting	(1,910.4)	(3,126.7)
Derivative Asset	3,448.1	441.5
Fair Value, Measurements, Recurring | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets, Netting	(1,745.2)	(2,937.2)
Derivative Asset	3,252.1	329.5
Fair Value, Measurements, Recurring | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets, Netting	(165.2)	(189.5)
Derivative Asset	196.0	112.0
Fair Value, Measurements, Recurring | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Liability, Netting	(4,199.6)	(2,205.4)
Derivative Liabilities	958.8	1,444.4
Fair Value, Measurements, Recurring | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Liability, Netting	(4,197.3)	(2,175.7)
Derivative Liabilities	512.5	1,079.5
Fair Value, Measurements, Recurring | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Liability, Netting	(2.3)	(6.0)
Derivative Liabilities	419.1	363.2
Fair Value, Measurements, Recurring | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Liability, Netting	0.0	(23.7)
Derivative Liabilities	27.2	1.7
Fair Value, Measurements, Recurring | LEVEL 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	7,367.5	3,622.2
Equity Securities and Money Market Investment		95.0
Fair Value, Measurements, Recurring | LEVEL 1 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Equity Securities and Money Market Investment	85.0
Derivative Assets, before Netting	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, before Netting	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, before Netting	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, before Netting	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, before Netting	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, before Netting	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, before Netting	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	21,634.0	19,467.6
Equity Securities and Money Market Investment		0.0
Fair Value, Measurements, Recurring | LEVEL 2 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Equity Securities and Money Market Investment	26.3
Derivative Assets, before Netting	5,358.5	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, before Netting	4,997.3	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, before Netting	361.2	301.5
Fair Value, Measurements, Recurring | LEVEL 2 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities, before Netting	5,131.2	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, before Netting	4,709.8	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, before Netting	421.4	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, before Netting	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
Equity Securities and Money Market Investment		0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Equity Securities and Money Market Investment	0.0
Derivative Assets, before Netting	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, before Netting	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, before Netting	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, before Netting	27.2	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, before Netting	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, before Netting	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, before Netting	27.2	25.4
U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	7,367.5	3,622.2
U.S. Government | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	7,367.5	3,622.2
U.S. Government | Fair Value, Measurements, Recurring | LEVEL 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	7,367.5	3,622.2
U.S. Government | Fair Value, Measurements, Recurring | LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
U.S. Government | Fair Value, Measurements, Recurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	297.6	295.8
Obligations of States and Political Subdivisions | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	297.6	295.8
Obligations of States and Political Subdivisions | Fair Value, Measurements, Recurring | LEVEL 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Obligations of States and Political Subdivisions | Fair Value, Measurements, Recurring | LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	297.6	295.8
Obligations of States and Political Subdivisions | Fair Value, Measurements, Recurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	13,288.9	11,553.0
Government Sponsored Agency | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	13,288.9	11,553.0
Government Sponsored Agency | Fair Value, Measurements, Recurring | LEVEL 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Government Sponsored Agency | Fair Value, Measurements, Recurring | LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	13,288.9	11,553.0
Government Sponsored Agency | Fair Value, Measurements, Recurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	296.8	264.4
Non-U.S. Government | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	296.8	264.4
Non-U.S. Government | Fair Value, Measurements, Recurring | LEVEL 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Non-U.S. Government | Fair Value, Measurements, Recurring | LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	296.8	264.4
Non-U.S. Government | Fair Value, Measurements, Recurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	163.8	279.5
Corporate Debt | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	163.8	279.5
Corporate Debt | Fair Value, Measurements, Recurring | LEVEL 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Corporate Debt | Fair Value, Measurements, Recurring | LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	163.8	279.5
Corporate Debt | Fair Value, Measurements, Recurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	230.9	347.1
Covered Bonds | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	230.9	347.1
Covered Bonds | Fair Value, Measurements, Recurring | LEVEL 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Covered Bonds | Fair Value, Measurements, Recurring | LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	230.9	347.1
Covered Bonds | Fair Value, Measurements, Recurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	4,583.1	2,899.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	4,583.1	2,899.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | Fair Value, Measurements, Recurring | LEVEL 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | Fair Value, Measurements, Recurring | LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	4,583.1	2,899.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds | Fair Value, Measurements, Recurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	2,182.7	2,962.6
Other Asset-Backed | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	2,182.7	2,962.6
Other Asset-Backed | Fair Value, Measurements, Recurring | LEVEL 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Other Asset-Backed | Fair Value, Measurements, Recurring | LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	2,182.7	2,962.6
Other Asset-Backed | Fair Value, Measurements, Recurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	590.2	865.3
Commercial Mortgage-Backed | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	590.2	865.3
Commercial Mortgage-Backed | Fair Value, Measurements, Recurring | LEVEL 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	0.0	0.0
Commercial Mortgage-Backed | Fair Value, Measurements, Recurring | LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	590.2	865.3
Commercial Mortgage-Backed | Fair Value, Measurements, Recurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale	$ 0.0	$ 0.0