Derivative Financial Instruments - Schedule of The Fair Values of Derivative Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	$ 11,054	$ 11,606
Fair value of derivative liabilities	11,557	11,852
Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	11,049	11,597
Fair value of derivative liabilities	11,173	11,467
Interest-rate contracts(6)
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	4	8
Fair value of derivative liabilities	96	100
Derivatives not designated as hedging instruments | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	11,014	11,478
Derivatives not designated as hedging instruments | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	11,302	11,645
Derivatives not designated as hedging instruments | Foreign exchange contracts | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	11,013	11,477
Derivatives not designated as hedging instruments | Foreign exchange contracts | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	11,014	11,361
Derivatives not designated as hedging instruments | Other derivative contracts | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	1	1
Derivatives not designated as hedging instruments | Other derivative contracts | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	288	284
Derivatives designated as hedging instruments | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	40	128
Derivatives designated as hedging instruments | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	255	207
Derivatives designated as hedging instruments | Foreign exchange contracts | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	36	120
Derivatives designated as hedging instruments | Foreign exchange contracts | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	159	107
Derivatives designated as hedging instruments | Interest-rate contracts(6) | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	4	8
Derivatives designated as hedging instruments | Interest-rate contracts(6) | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	$ 96	$ 100