Derivative Financial Instruments - Schedule of The Fair Values of Derivative Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	$ 17,382	$ 11,606
Fair value of derivative liabilities	17,934	11,852
Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	17,380	11,597
Fair value of derivative liabilities	17,558	11,467
Interest-rate contracts(6)
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	0	8
Fair value of derivative liabilities	106	100
Derivatives not designated as hedging instruments | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	17,334	11,478
Derivatives not designated as hedging instruments | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	17,740	11,645
Derivatives not designated as hedging instruments | Foreign exchange contracts | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	17,332	11,477
Derivatives not designated as hedging instruments | Foreign exchange contracts | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	17,470	11,361
Derivatives not designated as hedging instruments | Other derivative contracts | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	2	1
Derivatives not designated as hedging instruments | Other derivative contracts | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	270	284
Derivatives designated as hedging instruments | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	48	128
Derivatives designated as hedging instruments | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	194	207
Derivatives designated as hedging instruments | Foreign exchange contracts | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	48	120
Derivatives designated as hedging instruments | Foreign exchange contracts | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	88	107
Derivatives designated as hedging instruments | Interest-rate contracts(6) | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	0	8
Derivatives designated as hedging instruments | Interest-rate contracts(6) | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	$ 106	$ 100