Postretirement Benefit Plans (Postretirement Amounts Recognized in AOCI) (Details) - USD ($) $ in Millions	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2023
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial (losses) gains	$ (46)	$ (27)
Prior service credit	12	9
Total postretirement amounts recognized in accumulated other comprehensive loss	(34)	(18)	$ 34
Other Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial (losses) gains	37	37
Prior service credit	31	35
Total postretirement amounts recognized in accumulated other comprehensive loss	$ 68	$ 72	$ 74

Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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- Definition

Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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