Derivatives and Hedging Activities - Fair Values, Volume of Activity and Gain (Loss) Information Related to Derivative Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2020	Mar. 31, 2019
Derivatives, Fair Value [Line Items]
Notional Amount	$ 120,402	$ 126,077
Derivative assets, netting adjustments	(809)	(473)
Derivative Assets, net	2,065	778
Derivative liabilities, netting adjustments	(595)	(335)
Derivative Liabilities, net	649	156
Derivatives Not Designated as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	94,598	86,869
Derivative Assets	2,811	1,062
Derivative Liabilities	1,241	513
Interest Rate [Member] \| Derivatives Designated as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	25,804	39,208
Derivative Assets	66	191
Derivative Liabilities	3	20
Interest Rate [Member] \| Derivatives Not Designated as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	77,335	71,209
Derivative Assets	1,875	772
Derivative Liabilities	326	233
Foreign Exchange [Member] \| Derivatives Not Designated as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	6,340	6,572
Derivative Assets	164	67
Derivative Liabilities	157	60
Commodity [Member] \| Derivatives Not Designated as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	4,556	5,324
Derivative Assets	704	208
Derivative Liabilities	690	200
Credit [Member] \| Derivatives Not Designated as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	959	427
Derivative Assets	11	1
Derivative Liabilities	28	10
Other [Member] \| Derivatives Not Designated as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	5,408	3,337
Derivative Assets	57	14
Derivative Liabilities	40	10
Net Derivatives [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	120,402	126,077
Derivative Assets, net	2,068	780
Derivative Liabilities, net	649	198
Counterparty And Cash Collateral Netting [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	0	0
Derivative Assets, net	(3)	(2)
Derivative Liabilities, net	$ 0	$ (42)

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