Derivatives and Hedging Activities - Fair Values, Volume of Activity and Gain (Loss) Information Related to Derivative Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Derivatives, Fair Value [Line Items]
Notional Amount	$ 129,403	$ 126,077
Derivative assets, netting adjustments	(490)	(473)
Derivative Assets, net	1,943	778
Derivative liabilities, netting adjustments	(679)	(335)
Derivative Liabilities, net	167	156
Derivatives Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Notional Amount	95,183	86,869
Derivative Assets	2,365	1,062
Derivative Liabilities	841	513
Interest rate \| Derivatives Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Notional Amount	34,220	39,208
Derivative Assets	71	191
Derivative Liabilities	6	20
Interest rate \| Derivatives Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Notional Amount	75,507	71,209
Derivative Assets	1,756	772
Derivative Liabilities	300	233
Foreign exchange \| Derivatives Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Notional Amount	5,879	6,572
Derivative Assets	74	67
Derivative Liabilities	68	60
Commodity \| Derivatives Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Notional Amount	7,738	5,324
Derivative Assets	462	208
Derivative Liabilities	445	200
Credit \| Derivatives Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Notional Amount	487	427
Derivative Assets	3	1
Derivative Liabilities	15	10
Other
Derivatives, Fair Value [Line Items]
Derivative Assets	44
Other \| Derivatives Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Notional Amount	5,572	3,337
Derivative Assets	70	14
Derivative Liabilities	13	10
Net derivatives
Derivatives, Fair Value [Line Items]
Notional Amount	129,403	126,077
Derivative Assets, net	1,946	780
Derivative Liabilities, net	168	198
Other collateral
Derivatives, Fair Value [Line Items]
Notional Amount	0	0
Derivative Assets, net	(3)	(2)
Derivative Liabilities, net	$ (1)	$ (42)

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