Derivatives, Hedging Activities and Interest Expense - Derivative Activity Impact on Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2024	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	$ 73,589	$ 58,036
Fair value	1,238	1,937
Counterparty netting	(346)	(659)
Collateral held	(851)	(1,227)
Other assets [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	71,830	56,799
Fair value	1,149	1,888
Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	1,759	1,237
Fair value	89	49
Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	46,353	52,529
Fair value	970	1,470
Counterparty netting	(346)	(659)
Collateral posted	(601)	(794)
Other liabilities [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	38,920	46,082
Fair value	52	468
Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	7,433	6,447
Fair value	918	1,002
Carrying value [Member]
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other assets	41	51
Carrying value of derivative contracts – Other liabilities	$ 23	$ 17