Derivative Financial Instruments - Impact of Derivatives on Consolidated Statement of Income (Details) - Derivatives not designated as hedging instruments - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	$ 133	$ 116	$ 387	$ 361
Foreign exchange contracts | Trading services
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	170	152	549	485
Foreign exchange contracts | Processing fees and other revenue
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	0	(9)	0	(11)
Foreign exchange contracts | Interest expense
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	(3)	0	(18)	0
Interest-rate contracts | Trading services
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	(2)	(2)	(6)	7
Interest-rate contracts | Processing fees and other revenue
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	(1)	0	(1)	0
Other derivative contracts | Trading services
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	3	0	3	0
Other derivative contracts | Compensation and employee benefits
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	$ (34)	$ (25)	(140)	$ (120)
Swap | Foreign exchange contracts | Processing fees and other revenue
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income			$ 15