Derivative Financial Instruments - Impact of Derivatives on Consolidated Statement of Income (Details) - Derivatives not designated as hedging instruments - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income		$ 269	$ 509	$ 474
Foreign exchange contracts | Foreign exchange trading services
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income		630	723	632
Foreign exchange contracts | Software and processing fees
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income		0	0	(23)
Foreign exchange contracts | Interest expense
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income		(153)	(41)	0
Interest rate contracts | Foreign exchange trading services
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income		(3)	(6)	8
Interest rate contracts | Software and processing fees
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income		0	(1)	0
Other derivative contracts | Foreign exchange trading services
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income		0	5	0
Other derivative contracts | Compensation and employee benefits
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income		$ (205)	$ (171)	$ (143)
Swap agreements | Interest rate contracts | Software and processing fees
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	$ 15