Contingent Liabilities (Narrative) (Details) shares in Millions		1 Months Ended
	Jun. 28, 2018 USD ($)	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($) shares	Jan. 31, 2015 trust	Jun. 30, 2018 USD ($) shares	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($) shares
Loss Contingencies [Line Items]
Commitments and letters of credit					$ 25,700,000,000	$ 26,800,000,000
Standby letters of credit outstanding					$ 2,700,000,000	3,000,000,000
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
Remaining Visa shares held by Northern Trust (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
Indemnification Agreement
Loss Contingencies [Line Items]
Collateralized securities loaned, subject to indemnification					146,800,000,000	143,600,000,000
Credit loss liability					0	$ 0
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 Inc. | Visa Class B Litigation Case
Loss Contingencies [Line Items]
Escrow deposit	$ 600,000,000