Commitments and Contingent Liabilities (Narrative) (Details)		1 Months Ended	3 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended	81 Months Ended
	Mar. 05, 2024 EUR (€)	May 31, 2024 exchange_offer shares	Dec. 31, 2023 USD ($)	Sep. 30, 2024 USD ($) $ / shares shares	Sep. 30, 2024 USD ($) $ / shares	Dec. 31, 2015 trust	Sep. 30, 2024 USD ($) $ / shares
Loss Contingencies [Line Items]
Minimum securities lending related collateral percentage				100.00%	100.00%		100.00%
Commitments and letters of credit			$ 285,387,400,000	$ 301,721,300,000	$ 301,721,300,000		$ 301,721,300,000
Resell agreements, period end amounts excluding effects of agreements reported net by counterparty			27,700,000,000	11,500,000,000	11,500,000,000		11,500,000,000
Repurchase agreement counterparty, amount at risk			0	0	0		0
Loss contingency accrual				$ 0	$ 0		$ 0
Common stock, convertible, conversion price				1.5430	1.5430		1.5430
Securities Lent with Indemnification
Loss Contingencies [Line Items]
Commitments and letters of credit			140,539,100,000	$ 146,058,100,000	$ 146,058,100,000		$ 146,058,100,000
Indemnification Agreement
Loss Contingencies [Line Items]
Credit loss liability			0	0	0		0
FDIC Special Assessment
Loss Contingencies [Line Items]
Additional cost			84,600,000		14,700,000
Loss contingency accrual			$ 84,600,000	99,300,000	99,300,000		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							4,300,000,000
Visa Class A Common Stock
Loss Contingencies [Line Items]
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%
Visa Class B
Loss Contingencies [Line Items]
Carryover basis				0	0		0
Conversion of stock, amount converted				$ 876,000,000
Visa Class B-2 Shares
Loss Contingencies [Line Items]
Exchange offer program, shares received upon expiration (in shares) | shares		2,100,000
Visa Class C shares
Loss Contingencies [Line Items]
Exchange offer program, shares received upon expiration (in shares) | shares		819,500
Number of shares sold (in shares) | shares				794,200
Realized gain recorded					864,800,000
Equity securities				$ 27,800,000	$ 27,800,000		$ 27,800,000
Visa Class B Convertible Into Visa Class A | Visa
Loss Contingencies [Line Items]
Share price (in USD) | $ / shares				$ 274.95	$ 274.95		$ 274.95
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	$ 0		$ 0
Upper Limit
Loss Contingencies [Line Items]
Estimated loss				$ 20,000,000	$ 20,000,000		$ 20,000,000