Commitments and Contingent Liabilities (Narrative) (Details) 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 ($) shares	Dec. 31, 2015 trust	Mar. 31, 2022 USD ($) shares	Dec. 31, 2021 USD ($) shares
Loss Contingencies [Line Items]
Minimum securities lending related collateral percentage							100.00%
Commitments and letters of credit							$ 188,994,400,000	$ 201,976,600,000
Repurchase agreement counterparty, amount at risk							0	$ 0
Loss contingency accrual							$ 0
Visa Class B
Loss Contingencies [Line Items]
Visa redemption gain				$ 123,100,000	$ 99,900,000
Shares 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 serving 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
Visa Inc. | Visa Class B Litigation Case
Loss Contingencies [Line Items]
Escrow deposit	$ 250,000,000	$ 300,000,000	$ 600,000,000
Indemnification Agreement
Loss Contingencies [Line Items]
Credit loss liability							$ 0	$ 0