Derivatives, Hedging Activities and Interest Expense - Derivative Activity Impact on Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2025	Mar. 31, 2024
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	$ 71,466	$ 46,353
Fair value	748	970
Counterparty netting	(207)	(346)
Collateral held	(242)	(851)
Other assets [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	64,440	38,920
Fair value	31	52
Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	3,497	1,759
Fair value	132	89
Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	45,377	73,589
Fair value	495	1,238
Counterparty netting	(207)	(346)
Collateral posted	(514)	(601)
Other liabilities [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	41,880	71,830
Fair value	363	1,149
Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	7,026	7,433
Fair value	717	918
Carrying value [Member]
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other assets	46	41
Carrying value of derivative contracts – Other liabilities	$ 27	$ 23