Contingent Liabilities (Narrative) (Details) shares in Millions			1 Months Ended		3 Months Ended
	Sep. 27, 2019 USD ($)	Jun. 28, 2018 USD ($)	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($) shares	Mar. 31, 2016 trust	Sep. 30, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares
Loss Contingencies [Line Items]
Commitments and letters of credit						$ 26,400,000,000	$ 24,400,000,000
Standby letters of credit outstanding						$ 2,100,000,000	$ 2,400,000,000
Minimum securities lending related collateral percentage						100.00%
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						15,000,000
Visa Inc. | Visa Class B Litigation Case
Loss Contingencies [Line Items]
Escrow deposit	$ 300,000,000	$ 600,000,000
Indemnification Agreement
Loss Contingencies [Line Items]
Collateralized securities loaned, subject to indemnification						148,300,000,000	138,100,000,000
Credit loss liability						$ 0	$ 0