Derivative Instruments and Other Financial Instruments Used For Hedging - Notional and Fair Value Amounts (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments and Other Financial Instruments
Notional Amount	$ 201,094	$ 150,854
Fair Value, Asset Derivatives	871	1,322
Fair Value, Liability Derivatives	804	1,127
Interest rate contracts | Not designated as hedging instruments
Derivative Instruments and Other Financial Instruments
Notional Amount	189,478	132,214
Fair Value, Asset Derivatives	531	878
Fair Value, Liability Derivatives	600	646
Interest rate contracts | Cash flow hedges | Designated as hedging instrument
Derivative Instruments and Other Financial Instruments
Notional Amount	674	6,998
Fair Value, Asset Derivatives	6	2
Fair Value, Liability Derivatives	0	149
Interest rate contracts | Fair value hedges | Designated as hedging instrument
Derivative Instruments and Other Financial Instruments
Notional Amount	0	500
Fair Value, Asset Derivatives	0	0
Fair Value, Liability Derivatives	0	0
Commodity contracts | Not designated as hedging instruments
Derivative Instruments and Other Financial Instruments
Notional Amount	336	1,244
Fair Value, Asset Derivatives	25	50
Fair Value, Liability Derivatives	18	33
Foreign exchange contracts | Not designated as hedging instruments
Derivative Instruments and Other Financial Instruments
Notional Amount	8,475	7,053
Fair Value, Asset Derivatives	260	250
Fair Value, Liability Derivatives	168	146
Equity contracts | Not designated as hedging instruments
Derivative Instruments and Other Financial Instruments
Notional Amount	300	1,496
Fair Value, Asset Derivatives	22	139
Fair Value, Liability Derivatives	13	144
Other derivative contracts(2) | Not designated as hedging instruments
Derivative Instruments and Other Financial Instruments
Notional Amount	127	4
Fair Value, Asset Derivatives	0	0
Fair Value, Liability Derivatives	0	0
Trading | Not designated as hedging instruments
Derivative Instruments and Other Financial Instruments
Notional Amount	198,716	142,011
Fair Value, Asset Derivatives	838	1,317
Fair Value, Liability Derivatives	799	969
Other risk management | Not designated as hedging instruments
Derivative Instruments and Other Financial Instruments
Notional Amount	1,704	1,345
Fair Value, Asset Derivatives	27	3
Fair Value, Liability Derivatives	5	9
MUSA and Various other Non-Bank Subsidiaries
Derivative Instruments and Other Financial Instruments
Notional Amount	$ 3,600	$ 3,000