Segment and Related Information​ - Segment Operating Profit (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Adjusted EBITDA	$ 182	$ 172	$ 571	$ 483
Corporate expenses(1)	(7)	(6)	(26)	(15)
Stock-based compensation expense	(8)	(6)	(15)	(18)
Metal Price Lag	(21)	(16)	(27)	(30)
Provision for depreciation and amortization	(61)	(63)	(186)	(191)
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business	(14)	(3)	(612)	(61)
Other	(3)	(14)	(19)	(42)
Operating income (loss)	68	64	(314)	126
Interest expense	(26)	(22)	(74)	(97)
Other expenses, net	(15)	(27)	(52)	(69)
Provision (Benefit) for income taxes	(11)	(10)	81	(5)
Net Income (Loss) Attributable to Noncontrolling Interest	0	0	0	0
Net Income (Loss) Attributable to Parent	16	5	(359)	(45)
Operating segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Adjusted EBITDA	182	172	571	483
Provision for depreciation and amortization	(58)	(59)	(175)	(179)
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business	(3)	0	(6)	6
Operating segments | Rolled Products
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Adjusted EBITDA	155	138	493	388
Provision for depreciation and amortization	(48)	(48)	(145)	(147)
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business	0	1	1	(15)
Operating segments | Building and Construction Systems
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Adjusted EBITDA	34	40	97	107
Provision for depreciation and amortization	(4)	(5)	(13)	(14)
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business	0	0	1	(6)
Operating segments | Extrusions
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Adjusted EBITDA	7	6	19	12
Provision for depreciation and amortization	(6)	(6)	(17)	(18)
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business	$ (3)	$ 1	$ (6)	$ (27)