Commitments and Contingent Liabilities - Narrative (Detail) shares in Millions			1 Months Ended		12 Months Ended
	Sep. 27, 2019 USD ($)	Jun. 28, 2018 USD ($)	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($) shares	Dec. 31, 2015 trust	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares
Loss Contingencies [Line Items]
Minimum collateral maintained against fair value of client securities loaned plus accrued interest						100.00%
Accruals recorded for pending litigation or threatened legal actions or regulatory matters						$ 0
Indemnification Agreement
Loss Contingencies [Line Items]
Securities loaned and subject to indemnification						157,500,000,000	$ 138,100,000,000
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	$ 300,000,000	$ 600,000,000