Commitments and Contingent Liabilities - Narrative (Detail)		1 Months Ended	12 Months Ended
	Mar. 05, 2024 EUR (€)	May 31, 2024 exchange_offer shares	Dec. 31, 2024 USD ($) $ / shares shares	Dec. 31, 2023 USD ($)	Dec. 31, 2015 trust
Loss Contingencies [Line Items]
Minimum collateral maintained against fair value of client securities loaned plus accrued interest			100.00%
Fair value disclosure, off-balance sheet risks, amount, liability			$ 285,453,400,000	$ 285,387,400,000
Resell agreements, period end amounts excluding effects of agreements reported net by counterparty			0	27,700,000,000
Repurchase agreement counterparty, amount at risk			0	0
Liability recognized			0
Payment to escrow deposit			$ 4,300,000,000
Common stock, convertible, conversion price (in USD per share)			1.5430
Pre-tax gain on sale of equity investment			$ 68,100,000
Equity securities			26,300,000
Subsidiaries | Northern Trust Fiduciary Services (Guernsey) Limited
Loss Contingencies [Line Items]
Number of administrated trusts served as trustee on | trust					2
Financial Guarantee
Loss Contingencies [Line Items]
Fair value disclosure, off-balance sheet risks, amount, liability			144,543,700,000	140,539,100,000
Indemnification Agreement
Loss Contingencies [Line Items]
Credit loss liability			0	0
FDIC Special Assessment
Loss Contingencies [Line Items]
Expense recognized			14,700,000	$ 84,600,000
Liability recognized			99,300,000
Northern Trust Fiduciary Services (Guernsey) Limited
Loss Contingencies [Line Items]
Fine amount | €	€ 187,500
Visa Class B-2 Shares
Loss Contingencies [Line Items]
Exchange offer program, shares received upon expiration (in shares) | shares		2,100,000
Estimated value of investment based on current conversion rate			1,000,000,000.0
Remaining visa shares held by Northern Trust, original cost basis			0
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%
Pre-tax gain on sale of equity investment			870,400,000
Equity securities			$ 26,300,000
Number of shares sold (in shares) | shares			798,700
Visa Class A | Visa
Loss Contingencies [Line Items]
Closing stock price (in dollars per share) | $ / shares			$ 316.04
Lower Limit
Loss Contingencies [Line Items]
Possible loss in future periods in excess of accrual			$ 0
Upper Limit
Loss Contingencies [Line Items]
Possible loss in future periods in excess of accrual			$ 25,000,000