Commitments and Contingent Liabilities (Narrative) (Details)		1 Months Ended	3 Months Ended	12 Months Ended			87 Months Ended
	Mar. 05, 2024 EUR (€)	May 31, 2024 exchange_offer shares	Mar. 31, 2024 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2015 trust	Mar. 31, 2025 USD ($) $ / shares shares
Loss Contingencies [Line Items]
Minimum securities lending related collateral percentage							100.00%
Commitments and letters of credit				$ 285,453,400,000			$ 304,138,400,000
Repurchase agreement counterparty, amount at risk				0			0
Loss contingency accrual							$ 0
Common stock, convertible, conversion price							1.5342
Securities Lent with Indemnification
Loss Contingencies [Line Items]
Commitments and letters of credit				144,543,700,000			$ 155,667,300,000
Indemnification Agreement
Loss Contingencies [Line Items]
Credit loss liability				0			0
FDIC Special Assessment
Loss Contingencies [Line Items]
Additional cost			$ 12,500,000	$ 14,700,000	$ 84,600,000
Loss contingency accrual					$ 84,600,000		99,300,000
Northern Trust Fiduciary Services (Guernsey) Limited
Loss Contingencies [Line Items]
Litigation settlement, amount awarded to other party | €	€ 187,500
Visa
Loss Contingencies [Line Items]
Escrow deposit							4,600,000,000
Visa Class B-2 Shares
Loss Contingencies [Line Items]
Exchange offer program, shares received upon expiration (in shares) | shares		2,100,000
Carryover basis							0
Estimated value assuming conversion							$ 1,100,000,000
Visa Class C shares
Loss Contingencies [Line Items]
Exchange offer program, shares received upon expiration (in shares) | shares		819,500
Exchange offer program, number of exchange offers | exchange_offer		3
Exchange offer program, length of time between exchanges		12 months
Exchange offer program, claim reduction percentage		50.00%
Number of shares held (in shares) | shares							18,100
Equity securities							$ 25,300,000
Visa Class A | Visa
Loss Contingencies [Line Items]
Share price (in USD) | $ / shares							$ 350.46
Northern Trust Fiduciary Services (Guernsey) Limited | Subsidiaries
Loss Contingencies [Line Items]
Number of administrated trusts serving as trustee on | trust						2
Lower Limit
Loss Contingencies [Line Items]
Estimated loss							$ 0
Upper Limit
Loss Contingencies [Line Items]
Estimated loss							$ 25,000,000