Derivatives, Hedging Activities and Interest Expense - Derivative Activity Impact on Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2022	Mar. 31, 2022
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	$ 67,031	$ 65,921
Fair value	2,106	1,480
Counterparty netting	(809)	(761)
Collateral held	(1,243)	(658)
Other assets [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	66,902	64,327
Fair value	2,093	1,425
Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	129	1,594
Fair value	13	55
Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	40,801	28,005
Fair value	2,096	1,539
Counterparty netting	(809)	(761)
Collateral posted	(1,286)	(753)
Other liabilities [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	33,198	19,903
Fair value	779	822
Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	7,603	8,102
Fair value	1,317	717
Carrying value [Member]
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other assets	54	61
Carrying value of derivative contracts – Other liabilities	$ 1	$ 25