Segment, Major Customer and Major Supplier Information - Reconciliation from Segment Income to Consolidated Net Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting [Abstract]
Net income attributable to our common shareholder	$ 128	$ 157	$ 279	$ 313
Net income attributable to noncontrolling interests	1	0	0	0
Income tax provision	51	51	111	105
Income before income tax provision	180	208	390	418
Depreciation and amortization	141	136	281	267
Interest expense and amortization of debt issuance costs	72	78	144	155
Adjustment to reconcile proportional consolidation	12	11	25	25
Unrealized (gains) losses on change in fair value of derivative instruments, net	(9)	23	(16)	19
Realized losses (gains) on derivative instruments not included in Adjusted EBITDA	3	(1)	5	(4)
Loss on extinguishment of debt, net	0	5	0	5
Restructuring and impairment expenses, net	21	4	40	7
Loss on sale or disposal of assets, net	1	0	2	0
Metal price lag	(21)	22	(14)	17
Loss from catastrophes	61	0	101	0
Other, net	1	(2)	4	(4)
Adjusted EBITDA	462	$ 484	962	$ 905
Restructuring and impairment expenses, net	$ 17		$ 19