Fair Value Measurements (Recurring Basis Hierarchy Leveling) (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	$ 36,913.2	$ 34,036.5
Derivative Assets	3,857.7	2,094.3
NETTING	(2,179.8)	(1,779.2)
Derivative Assets	1,677.9	315.1
Derivative Liabilities	3,793.5	2,409.6
NETTING	(2,757.4)	(1,175.7)
Derivative Liabilities	1,036.1	1,233.9
Cross Product Collateral Adjustment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(245.4)	(1,200.0)
NETTING	(823.0)	(550.6)
U.S. Governments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	7,971.9	8,172.4
Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	311.9	313.1
Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	18,529.1	16,567.5
Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	311.6	527.2
Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	66.1	64.4
Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	275.0	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	5,014.7	4,984.3
CLOs
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	3,293.4	2,154.9
Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	703.4	570.2
Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	436.1	409.0
LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	28,900.0	25,900.0
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	36,913.2	34,036.5
Equity Securities	159.0	212.4
Fair Value, Measurements, Recurring | U.S. Governments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	7,971.9	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	311.9	313.1
Fair Value, Measurements, Recurring | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	18,529.1	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	311.6	527.2
Fair Value, Measurements, Recurring | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	66.1	64.4
Fair Value, Measurements, Recurring | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	275.0	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	5,014.7	4,984.3
Fair Value, Measurements, Recurring | CLOs
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	3,293.4	2,154.9
Fair Value, Measurements, Recurring | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	703.4	570.2
Fair Value, Measurements, Recurring | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	436.1	409.0
Fair Value, Measurements, Recurring | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(2,179.8)	(1,779.2)
Derivative Assets	1,677.9	315.1
Fair Value, Measurements, Recurring | Other Assets | Money market investment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Equity Securities	85.0	85.0
Fair Value, Measurements, Recurring | Other Assets | Seed Capital Investments, Certain Funds Managed by Northern Trust
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Equity Securities	61.1	112.5
Fair Value, Measurements, Recurring | Other Assets | Visa Class C shares
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Equity Securities	12.9	14.9
Fair Value, Measurements, Recurring | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(2,088.5)	(1,696.1)
Derivative Assets	1,665.2	292.7
Fair Value, Measurements, Recurring | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(91.3)	(82.4)
Derivative Assets	12.7	22.4
Fair Value, Measurements, Recurring | Other Assets | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING		(0.7)
Derivative Assets		0.0
Fair Value, Measurements, Recurring | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(2,757.4)	(1,175.7)
Derivative Liabilities	1,036.1	1,233.9
Fair Value, Measurements, Recurring | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(2,753.8)	(1,139.4)
Derivative Liabilities	885.6	1,108.5
Fair Value, Measurements, Recurring | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	(3.6)	(5.0)
Derivative Liabilities	126.8	125.4
Fair Value, Measurements, Recurring | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
NETTING	0.0	(31.3)
Derivative Liabilities	23.7	0.0
Fair Value, Measurements, Recurring | LEVEL 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	7,971.9	8,172.4
Equity Securities	85.0	85.0
Fair Value, Measurements, Recurring | LEVEL 1 | U.S. Governments
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	7,971.9	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	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 1 | Other Assets
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 | 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 Assets | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets		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	28,941.3	25,864.1
Equity Securities	74.0	127.4
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	311.9	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	18,529.1	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	311.6	527.2
Fair Value, Measurements, Recurring | LEVEL 2 | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	66.1	64.4
Fair Value, Measurements, Recurring | LEVEL 2 | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	275.0	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	5,014.7	4,984.3
Fair Value, Measurements, Recurring | LEVEL 2 | CLOs
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	3,293.4	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	703.4	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	436.1	409.0
Fair Value, Measurements, Recurring | LEVEL 2 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	3,857.7	2,094.3
Fair Value, Measurements, Recurring | LEVEL 2 | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	3,753.7	1,988.8
Fair Value, Measurements, Recurring | LEVEL 2 | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	104.0	104.8
Fair Value, Measurements, Recurring | LEVEL 2 | Other Assets | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets		0.7
Fair Value, Measurements, Recurring | LEVEL 2 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	3,771.1	2,379.9
Fair Value, Measurements, Recurring | LEVEL 2 | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	3,639.4	2,247.9
Fair Value, Measurements, Recurring | LEVEL 2 | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	130.4	130.4
Fair Value, Measurements, Recurring | LEVEL 2 | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	1.3	1.6
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
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	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
Fair Value, Measurements, Recurring | LEVEL 3 | Other Assets
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 | 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 Assets | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets		0.0
Fair Value, Measurements, Recurring | LEVEL 3 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	22.4	29.7
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	$ 22.4	$ 29.7