Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning of period	$ 1,065	$ 1,084	$ 1,003	$ 1,047
OCI, before reclassifications and income taxes		(8)		(7)
Income taxes		2		2
OCI before reclassifications, net of income taxes		(6)		(5)
Amounts reclassified from AOCI to earnings, before income taxes	0	(3)	(1)	(7)
Income taxes	0	1	0	2
Amounts reclassified from AOCI to earnings, net of income taxes	0	(2)	(1)	(5)
Current-period OCI, net of income taxes	0	(8)	(1)	(10)
End of period	996	1,201	996	1,201
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning of period	(30)	8	(29)	10
End of period	(30)	0	(30)	0
Post-retirement plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning of period	(28)	2	(28)	2
OCI, before reclassifications and income taxes		0		0
Income taxes		0		0
OCI before reclassifications, net of income taxes		0		0
Amounts reclassified from AOCI to earnings, before income taxes	1	0	1	0
Income taxes	0	0	0	0
Amounts reclassified from AOCI to earnings, net of income taxes	1	0	1	0
Current-period OCI, net of income taxes	1	0	1	0
End of period	(27)	2	(27)	2
Derivatives designated as cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning of period	(2)	6	(1)	8
OCI, before reclassifications and income taxes		(8)		(7)
Income taxes		2		2
OCI before reclassifications, net of income taxes		(6)		(5)
Amounts reclassified from AOCI to earnings, before income taxes	(1)	(3)	(2)	(7)
Income taxes	0	1	0	2
Amounts reclassified from AOCI to earnings, net of income taxes	(1)	(2)	(2)	(5)
Current-period OCI, net of income taxes	(1)	(8)	(2)	(10)
End of period	$ (3)	$ (2)	$ (3)	$ (2)

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