Fair Value Measurements (Recurring Basis Hierarchy Leveling) (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	$ 38,123.1	$ 34,036.5
Derivative Assets	3,375.1	2,094.3
NETTING	(1,830.4)	(1,779.2)
Derivative Assets	1,544.7	315.1
Derivative Liabilities	3,321.3	2,409.6
NETTING	(2,752.4)	(1,175.7)
Derivative Liabilities	568.9	1,233.9
Cross Product Collateral Adjustment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(202.7)	(1,200.0)
NETTING	(1,124.7)	(550.6)
U.S. Governments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	7,883.3	8,172.4
Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale		313.1
Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	19,302.8	16,567.5
Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	408.5	527.2
Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	271.4	64.4
Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	275.7	273.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	4,670.4	4,984.3
CLOs
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	3,453.7	2,154.9
Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	1,345.7	570.2
Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	511.6	409.0
LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	30,200.0	25,900.0
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	38,123.1	34,036.5
Equity Securities	560.9	212.4
NETTING	(1,830.4)	(1,779.2)
Derivative Assets	1,544.7	315.1
NETTING	(2,752.4)	(1,175.7)
Derivative Liabilities	568.9	1,233.9
Fair Value, Measurements, Recurring | Money market investment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Equity Securities	110.0	85.0
Fair Value, Measurements, Recurring | Visa Class C shares
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Equity Securities	370.7	14.9
Fair Value, Measurements, Recurring | Seed Capital Investments, Certain Funds Managed by Northern Trust
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Equity Securities	80.2	112.5
Fair Value, Measurements, Recurring | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(1,768.9)	(1,696.1)
Derivative Assets	1,525.7	292.7
NETTING	(2,729.3)	(1,139.4)
Derivative Liabilities	430.1	1,108.5
Fair Value, Measurements, Recurring | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(61.2)	(82.4)
Derivative Assets	19.0	22.4
NETTING	0.0	(5.0)
Derivative Liabilities	138.8	125.4
Fair Value, Measurements, Recurring | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(0.3)	(0.7)
Derivative Assets	0.0	0.0
NETTING	(23.1)	(31.3)
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | U.S. Governments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	7,883.3	8,172.4
Fair Value, Measurements, Recurring | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale		313.1
Fair Value, Measurements, Recurring | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	19,302.8	16,567.5
Fair Value, Measurements, Recurring | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	408.5	527.2
Fair Value, Measurements, Recurring | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	271.4	64.4
Fair Value, Measurements, Recurring | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	275.7	273.5
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	4,670.4	4,984.3
Fair Value, Measurements, Recurring | CLOs
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	3,453.7	2,154.9
Fair Value, Measurements, Recurring | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	1,345.7	570.2
Fair Value, Measurements, Recurring | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	511.6	409.0
Fair Value, Measurements, Recurring | LEVEL 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	7,883.3	8,172.4
Equity Securities	110.0	85.0
Derivative Assets	0.0	0.0
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	0.0	0.0
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	0.0	0.0
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	0.0	0.0
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 1 | U.S. Governments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	7,883.3	8,172.4
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
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 | CLOs
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 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	30,239.8	25,864.1
Equity Securities	450.9	127.4
Derivative Assets	3,375.1	2,094.3
Derivative Liabilities	3,298.3	2,379.9
Fair Value, Measurements, Recurring | LEVEL 2 | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	3,294.6	1,988.8
Derivative Liabilities	3,159.4	2,247.9
Fair Value, Measurements, Recurring | LEVEL 2 | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	80.2	104.8
Derivative Liabilities	138.8	130.4
Fair Value, Measurements, Recurring | LEVEL 2 | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	0.3	0.7
Derivative Liabilities	0.1	1.6
Fair Value, Measurements, Recurring | LEVEL 2 | U.S. Governments
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		313.1
Fair Value, Measurements, Recurring | LEVEL 2 | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	19,302.8	16,567.5
Fair Value, Measurements, Recurring | LEVEL 2 | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	408.5	527.2
Fair Value, Measurements, Recurring | LEVEL 2 | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	271.4	64.4
Fair Value, Measurements, Recurring | LEVEL 2 | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	275.7	273.5
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	4,670.4	4,984.3
Fair Value, Measurements, Recurring | LEVEL 2 | CLOs
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	3,453.7	2,154.9
Fair Value, Measurements, Recurring | LEVEL 2 | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	1,345.7	570.2
Fair Value, Measurements, Recurring | LEVEL 2 | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	511.6	409.0
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	0.0	0.0
Derivative Assets	0.0	0.0
Derivative Liabilities	23.0	29.7
Fair Value, Measurements, Recurring | LEVEL 3 | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	0.0	0.0
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	0.0	0.0
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	0.0	0.0
Derivative Liabilities	23.0	29.7
Fair Value, Measurements, Recurring | LEVEL 3 | U.S. Governments
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
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 | CLOs
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