Derivative Financial Instruments - Impact of Derivatives on Consolidated Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Net interest income	$ 673	$ 643	$ 673	$ 643
Processing fees and other	191	77
Derivatives not designated as hedging instruments
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	46	103
Derivatives not designated as hedging instruments | Foreign exchange contracts | Foreign exchange trading services
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	160	184
Derivatives not designated as hedging instruments | Foreign exchange contracts | Interest expense
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	(39)	(15)
Derivatives not designated as hedging instruments | Interest rate contracts | Foreign exchange trading services
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	(1)	(2)
Derivatives not designated as hedging instruments | Other derivative contracts | Foreign exchange trading services
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	0	1
Derivatives not designated as hedging instruments | Other derivative contracts | Compensation and employee benefits
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	(74)	(65)
Effect of Change
Derivative [Line Items]
Net interest income		15
Net interest income
Derivative [Line Items]
Net interest income	673	643
Net interest income | Effect of Change
Derivative [Line Items]
Net interest income		15
Processing fees and other
Derivative [Line Items]
Processing fees and other	$ 191	77
Processing fees and other | Effect of Change
Derivative [Line Items]
Processing fees and other		$ (15)