Contingent Liabilities (Narrative) (Details) shares in Millions		1 Months Ended			3 Months Ended		12 Months Ended
	Jan. 14, 2014 case	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($) shares	Jan. 31, 2015 trust	Sep. 30, 2016 USD ($) shares	Jun. 30, 2016 USD ($) shares	Dec. 31, 2013 USD ($) lawsuit	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) shares
Loss Contingencies [Line Items]
Commitments and letters of credit					$ 34,700,000,000			$ 37,200,000,000
Standby letters of credit outstanding					$ 4,000,000,000			4,000,000,000
Minimum securities lending related collateral percentage					100.00%
Legal settlement charge					$ (3,500,000)	$ (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
Shares issued | shares		1.1	1.0			1.1
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					103,600,000,000			94,500,000,000
Credit loss liability					0			$ 0
Upper Limit
Loss Contingencies [Line Items]
Possible loss in future periods in excess of accrual					35,000,000
Pending Litigation
Loss Contingencies [Line Items]
Loss contingency accrual					0				0
Threatened Litigation
Loss Contingencies [Line Items]
Loss contingency accrual					$ 0				$ 0