Commitments and Contingent Liabilities - Narrative (Detail) shares in Millions		1 Months Ended		12 Months Ended
	Sep. 13, 2023 exchangeOffer	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($) shares	Dec. 31, 2023 USD ($) shares	Dec. 31, 2015 trust	Dec. 31, 2022 USD ($) shares
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				$ 285,387,400,000		$ 179,758,000,000
Repurchase agreement counterparty, amount at risk				0		0
Resell agreements, period end amounts excluding effects of agreements reported net by counterparty				27,700,000,000		496,800,000
Accruals recorded for pending litigation or threatened legal actions or regulatory matters				0
Payment to escrow deposit				$ 2,800,000,000
Common stock, convertible, conversion price (in USD per share)				1.5875
Financial Guarantee
Loss Contingencies [Line Items]
Fair value disclosure, off-balance sheet risks, amount, liability				$ 140,539,100,000		130,311,000,000
Indemnification Agreement
Loss Contingencies [Line Items]
Credit loss liability				$ 0		$ 0
Visa Class B
Loss Contingencies [Line Items]
Redemption gain		$ 123,100,000	$ 99,900,000
Common stock issued (in shares) | shares		1.1	1.0
Common stock shares held (in shares) | shares				4.1		4.1
Remaining visa shares held by Northern Trust, original cost basis				$ 0		$ 0
Visa Class C Convertible Into Visa Class A
Loss Contingencies [Line Items]
Potential exchange offer program, number of exchange offers | exchangeOffer	3
Potential exchange offer program, length of time between exchanges	12 months
Potential exchange offer program, claim reduction percentage	50.00%
Visa Class C Convertible Into Visa Class A | Period One
Loss Contingencies [Line Items]
Potential exchange offer program, conversion period	45 days
Potential exchange offer program, exchange ratio	0.3333
Visa Class C Convertible Into Visa Class A | Period Two
Loss Contingencies [Line Items]
Potential exchange offer program, conversion period	90 days
Potential exchange offer program, exchange ratio	0.6666
Visa Class C Convertible Into Visa Class A | Period Three
Loss Contingencies [Line Items]
Potential exchange offer program, conversion period	90 days
Northern Trust Fiduciary Services (Guernsey) Limited | Subsidiaries
Loss Contingencies [Line Items]
Number of administrated trusts served as trustee on | trust					2
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