Derivative Financial Instruments - Impact of Derivatives on Consolidated Statement of Income (Details) - Derivatives not designated as hedging instruments - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	$ 150	$ 140	$ 254	$ 245
Foreign exchange contracts | Trading services
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	195	170	379	333
Foreign exchange contracts | Processing fees and other revenue
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	0	0	(15)	0
Interest-rate contracts(6) | Trading services
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	(2)	8	(4)	9
Interest-rate contracts(6) | Processing fees and other revenue
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	0	(9)	0	(2)
Other derivative contracts | Trading services
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	(1)	0	0	0
Other derivative contracts | Compensation and employee benefits
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	$ (42)	$ (29)	(106)	$ (95)
Swap | Foreign exchange contracts | Processing fees and other revenue
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income			$ 15