Derivative Instruments - Cumulative Fair Value Hedging Adjustments (Details) $ in Millions	Sep. 30, 2018 USD ($)
Commodity contracts
Assets
Carrying amount of the hedged items	$ 4,600
Long-term debt
Liabilities
Carrying amount of the hedged items	135,239
Cumulative amount of fair value hedging adjustments included in the carrying amount of hedged items:
Active hedging relationships	(2,693)
Discontinued hedging relationships	(5)
Total	(2,698)
Long-term debt | Not designated as hedges
Liabilities
Carrying amount of the hedged items	7,200
Beneficial interests issued by consolidated VIEs
Liabilities
Carrying amount of the hedged items	6,976
Cumulative amount of fair value hedging adjustments included in the carrying amount of hedged items:
Active hedging relationships	0
Discontinued hedging relationships	(42)
Total	(42)
Investment securities - AFS
Assets
Carrying amount of the hedged items	47,896
Cumulative amount of fair value hedging adjustments included in the carrying amount of hedged items:
Active hedging relationships	(2,292)
Discontinued hedging relationships	438
Total	(1,854)
Investment securities - AFS | Not designated as hedges
Assets
Carrying amount of the hedged items	$ 14,700