Contingent Liabilities (Narrative) (Details) shares in Millions		1 Months Ended	12 Months Ended
	Jan. 14, 2014 case	Jan. 31, 2015 trust	Dec. 31, 2013 USD ($) lawsuit	Mar. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Commitments and letters of credit				$ 37,400,000,000	$ 37,200,000,000
Standby letters of credit outstanding				$ 4,300,000,000
Minimum securities lending related collateral percentage				100.00%
Legal settlement charge			$ 19,200,000
Number of lawsuits settled, charges that have been recorded | lawsuit			2
Number of opt-out cases filed by merchants | case	30
Remaining Visa shares held by Northern Trust, original cost basis				$ 0
Visa Class B
Loss Contingencies [Line Items]
Remaining Visa shares held by Northern Trust | shares				5.2
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				$ 97,200,000,000	94,500,000,000
Credit loss liability				0	$ 0
Upper Limit
Loss Contingencies [Line Items]
Possible loss in future periods in excess of accrual				$ 140,000,000