Derivatives, Hedging Activities and Interest Expense - Derivative Activity Impact on Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2024	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	$ 62,039	$ 73,589
Fair value	520	1,238
Counterparty netting	(175)	(346)
Collateral held	(305)	(851)
Other assets [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	59,915	71,830
Fair value	470	1,149
Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	2,124	1,759
Fair value	50	89
Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	55,877	46,353
Fair value	1,008	970
Counterparty netting	(175)	(346)
Collateral posted	(793)	(601)
Other liabilities [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	48,223	38,920
Fair value	38	52
Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	7,654	7,433
Fair value	970	918
Carrying value [Member]
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other assets	40	41
Carrying value of derivative contracts – Other liabilities	$ 40	$ 23