Derivatives, Hedging Activities and Interest Expense - Derivative Activity Impact on Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Jun. 30, 2025	Mar. 31, 2025
Other assets [Member]
Derivatives Fair Value [Line Items]
Notional	$ 42,668	$ 45,377
Fair value	844	495
Counterparty netting	(243)	(207)
Collateral held	(526)	(242)
Other assets [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	37,665	41,880
Fair value	294	363
Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	5,003	3,497
Fair value	550	132
Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional		71,466
Notional	69,561
Fair value	395	748
Counterparty netting	(243)	(207)
Collateral posted	(145)	(514)
Other liabilities [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	63,750	64,440
Fair value	30	31
Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	5,811	7,026
Fair value	365	717
Carrying value [Member]
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other assets	$ 75	$ 46
Derivative Asset Statement Of Financial Position	Other assets	Other assets
Carrying value of derivative contracts – Other liabilities	$ 7	$ 27
Derivative liability Statement Of Financial Position	Other liabilities	Other liabilities