Contingent Liabilities (Narrative) (Details) shares in Millions		1 Months Ended	3 Months Ended		12 Months Ended
	Jan. 14, 2014 case	Jan. 31, 2015 trust	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($) shares	Dec. 31, 2013 USD ($) lawsuit	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Commitments and letters of credit			$ 36,400,000,000			$ 37,200,000,000
Standby letters of credit outstanding			$ 4,100,000,000			4,100,000,000
Minimum securities lending related collateral percentage			100.00%
Legal settlement charge			$ (46,500,000)		$ 19,200,000
Number of lawsuits settled, charges that have been recorded | lawsuit					2
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
Remaining Visa shares held by Northern Trust | shares			4.1	5.2
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			97,800,000,000			94,500,000,000
Credit loss liability			0			$ 0
Upper Limit
Loss Contingencies [Line Items]
Possible loss in future periods in excess of accrual			$ 90,000,000