Contingent Liabilities (Narrative) (Details) shares in Millions		1 Months Ended
	Jan. 14, 2014 case	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($) shares	Jan. 31, 2015 trust	Sep. 30, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($) shares
Loss Contingencies [Line Items]
Commitments and letters of credit					$ 27,800,000,000	$ 32,800,000,000
Standby letters of credit outstanding					$ 3,300,000,000	3,800,000,000
Minimum securities lending related collateral percentage					100.00%
Number of opt-out cases filed by merchants | case	30
Visa Class B
Loss Contingencies [Line Items]
Visa redemption gain		$ 123,100,000	$ 99,900,000
Shares issued | shares		1.1	1.0
Remaining Visa shares held by Northern Trust | 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					139,300,000,000	102,300,000,000
Credit loss liability					0	$ 0
Upper Limit
Loss Contingencies [Line Items]
Possible loss in future periods in excess of accrual					30,000,000
Pending Litigation
Loss Contingencies [Line Items]
Loss contingency accrual					0		0
Threatened Litigation
Loss Contingencies [Line Items]
Loss contingency accrual					$ 0		$ 0