Commitments and Contingent Liabilities - Narrative (Detail) shares in Millions				1 Months Ended		12 Months Ended
	Dec. 29, 2021 USD ($)	Sep. 27, 2019 USD ($)	Jun. 28, 2018 USD ($)	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($) shares	Dec. 31, 2015 trust	Dec. 31, 2021 USD ($) shares	Dec. 31, 2020 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							$ 201,976,600,000	$ 188,462,700,000
Repurchase agreement counterparty, amount at risk							0	0
Resell agreements, period end amounts excluding effects of agreements reported net by counterparty							0	0
Accruals recorded for pending litigation or threatened legal actions or regulatory matters							0
Financial Guarantee
Loss Contingencies [Line Items]
Fair value disclosure, off-balance sheet risks, amount, liability							170,445,300,000	157,478,000,000
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
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							$ 20,000,000
Visa Class B Litigation Case | Visa Inc.
Loss Contingencies [Line Items]
Payment to escrow deposit	$ 250,000,000	$ 300,000,000	$ 600,000,000