Derivative Financial Instruments - Schedule of The Fair Values of Derivative Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	$ 11,606	$ 16,552
Fair value of derivative liabilities	11,852	16,684
Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	11,597	16,484
Fair value of derivative liabilities	11,467	15,956
Interest-rate contracts(6)
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	8	68
Fair value of derivative liabilities	100	348
Derivatives not designated as hedging instruments | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	11,478	15,982
Derivatives not designated as hedging instruments | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	11,645	16,261
Derivatives not designated as hedging instruments | Foreign exchange contracts | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	11,477	15,982
Derivatives not designated as hedging instruments | Foreign exchange contracts | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	11,361	15,881
Derivatives not designated as hedging instruments | Other derivative contracts | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	1	0
Derivatives not designated as hedging instruments | Other derivative contracts | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	284	380
Derivatives designated as hedging instruments | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	128	570
Derivatives designated as hedging instruments | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	207	423
Derivatives designated as hedging instruments | Foreign exchange contracts | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	120	502
Derivatives designated as hedging instruments | Foreign exchange contracts | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	107	75
Derivatives designated as hedging instruments | Interest-rate contracts(6) | Other Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	8	68
Derivatives designated as hedging instruments | Interest-rate contracts(6) | Other liabilities
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	$ 100	$ 348