Commitments and Contingent Liabilities - Narrative (Detail) shares in Millions	1 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($) shares	Dec. 31, 2022 USD ($) shares	Dec. 31, 2015 trust	Dec. 31, 2021 USD ($) 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			$ 179,758,000,000		$ 201,976,600,000
Repurchase agreement counterparty, amount at risk			0		0
Resell agreements, period end amounts excluding effects of agreements reported net by counterparty			496,800,000		0
Accruals recorded for pending litigation or threatened legal actions or regulatory matters			0
Payment to escrow deposit			2,100,000,000
Total Residential Real Estate			6,413,500,000		6,319,900,000
Financial Guarantee
Loss Contingencies [Line Items]
Fair value disclosure, off-balance sheet risks, amount, liability			130,311,000,000		170,445,300,000
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