Derivative Instruments - Notional Balances and Fair Values of Derivatives (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Mar. 31, 2023
Derivatives, Fair Value [Line Items]
Assets	$ 842	$ 1,133
Collateral posted/held	5	0
Counterparty netting adjustment	(818)	(1,000)
Net derivative assets	29	133
Liabilities	1,393	1,736
Collateral posted/held	18	37
Counterparty netting adjustment	(818)	(1,000)
Net derivative liabilities	593	773
Interest rate swaps
Derivatives, Fair Value [Line Items]
Notional balances	59,486	55,974
Assets	777	1,133
Liabilities	892	1,062
Cross currency swaps
Derivatives, Fair Value [Line Items]
Notional balances	5,737	4,328
Assets	65	0
Liabilities	$ 501	$ 674