Segment and Related Information​ - Operating Results (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]
Total sales	$ 1,890	$ 1,415	$ 5,366	$ 4,213
Segment Adjusted EBITDA	182	172	571	483
Provision for depreciation and amortization	61	63	186	191
Restructuring and other charges (E)	14	3	612	61
2020 Restructuring Programs
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Restructuring and other charges (E)		3		61
One-time Termination Benefits
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Restructuring and other charges (E)	5		573
One-time Termination Benefits | 2020 Restructuring Programs
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Restructuring and other charges (E)		3		58
Intersegment sales
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total sales	10	4	27	15
Intersegment sales | Rolled Products
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total sales	9	3	26	14
Intersegment sales | Building and Construction Systems
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total sales	0	0	0	0
Intersegment sales | Extrusions
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total sales	1	1	1	1
Operating segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total sales	1,900	1,419	5,393	4,232
Segment Adjusted EBITDA	182	172	571	483
Provision for depreciation and amortization	58	59	175	179
Restructuring and other charges (E)	3	0	6	(6)
Operating segments | Rolled Products
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total sales	1,568	1,095	4,423	3,208
Segment Adjusted EBITDA	155	138	493	388
Provision for depreciation and amortization	48	48	145	147
Restructuring and other charges (E)	0	(1)	(1)	15
Operating segments | Building and Construction Systems
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total sales	257	241	750	727
Segment Adjusted EBITDA	34	40	97	107
Provision for depreciation and amortization	4	5	13	14
Restructuring and other charges (E)	0	0	(1)	6
Operating segments | Extrusions
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total sales	75	83	220	297
Segment Adjusted EBITDA	7	6	19	12
Provision for depreciation and amortization	6	6	17	18
Restructuring and other charges (E)	3	(1)	6	27
Third-party sales | External Sales Including Reconciling Items
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total sales	1,890	1,415	5,366	4,217
Third-party sales | External Sales Including Reconciling Items | Rolled Products
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total sales	1,559	1,092	4,397	3,194
Third-party sales | External Sales Including Reconciling Items | Building and Construction Systems
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total sales	257	241	750	727
Third-party sales | External Sales Including Reconciling Items | Extrusions
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total sales	$ 74	$ 82	$ 219	$ 296