Derivatives, Hedging Activities and Interest Expense - Derivative Activity Impact on Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2023	Mar. 31, 2023
Derivatives Fair Value [Line Items]
Derivative Asset Statement Of Financial Position	Other assets	Other assets
Derivative liability Statement Of Financial Position	Other liabilities	Other liabilities
Other assets [Member]
Derivatives Fair Value [Line Items]
Notional	$ 64,193	$ 58,036
Fair value	1,420	1,937
Counterparty netting	(390)	(659)
Collateral held	(985)	(1,227)
Other assets [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	61,513	56,799
Fair value	1,279	1,888
Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	2,680	1,237
Fair value	141	49
Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	54,273	52,529
Fair value	852	1,470
Counterparty netting	(390)	(659)
Collateral posted	(413)	(794)
Other liabilities [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	47,880	46,082
Fair value	104	468
Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	6,393	6,447
Fair value	748	1,002
Carrying value [Member]
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other assets	45	51
Carrying value of derivative contracts – Other liabilities	$ 49	$ 17