SEGMENT INFORMATION - Segment Revenue Segment Cost and Adjusted EBITDA (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Segment Reporting Information [Line Items]
External revenue	$ 2,461	$ 1,979
Segment Adjusted EBITDA	262	147
A&T
Segment Reporting Information [Line Items]
External revenue	583	435
Cost of metal	(280)	(185)
Production costs	(174)	(146)
Other segment expenses	(27)	(22)
Segment Adjusted EBITDA	102	82
A&T \| Segment revenue
Segment Reporting Information [Line Items]
External revenue	609	468
A&T \| Inter-segment elimination
Segment Reporting Information [Line Items]
External revenue	(26)	(33)
P&ARP
Segment Reporting Information [Line Items]
External revenue	1,474	1,184
Cost of metal	(1,039)	(859)
Production costs	(254)	(239)
Other segment expenses	(30)	(26)
Segment Adjusted EBITDA	151	60
P&ARP \| Segment revenue
Segment Reporting Information [Line Items]
External revenue	1,477	1,187
P&ARP \| Inter-segment elimination
Segment Reporting Information [Line Items]
External revenue	(3)	(3)
AS&I
Segment Reporting Information [Line Items]
External revenue	402	360
Cost of metal	(236)	(214)
Production costs	(117)	(107)
Other segment expenses	(25)	(23)
Segment Adjusted EBITDA	24	16
AS&I \| Segment revenue
Segment Reporting Information [Line Items]
External revenue	415	381
AS&I \| Inter-segment elimination
Segment Reporting Information [Line Items]
External revenue	(13)	(21)
H&C
Segment Reporting Information [Line Items]
External revenue	2	0
Cost of metal	2	2
Production costs	(2)	(2)
Other segment expenses	(17)	(11)
Segment Adjusted EBITDA	(15)	(11)
H&C \| Segment revenue
Segment Reporting Information [Line Items]
External revenue	2	0
H&C \| Inter-segment elimination
Segment Reporting Information [Line Items]
External revenue	$ 0	$ 0

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