Segment and Related Information​ - Segment Operating Profit (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Segment Adjusted EBITDA	$ 103	$ 223	$ 320	$ 393
Stock-based compensation expense	(5)	(12)	(12)	(18)
Restructuring and other charges	(77)	(38)	(58)	(40)
Other	(13)	(45)	(28)	(43)
Operating (loss) income	(67)	52	85	132
Interest expense	(40)	(29)	(75)	(57)
Other (expenses) income, net	(16)	(10)	(42)	4
(Benefit) Provision for income taxes (H)	31	(8)	0	(33)
Net Income (Loss) Attributable to Noncontrolling Interest	0	0	0	0
Income before income taxes	(123)	13	(32)	79
Non-service costs — Pension and OPEB (G)	24	28	50	56
Net Income (Loss) Attributable to Parent	(92)	5	(32)	46
Provision for depreciation and amortization	(68)	(64)	(128)	(127)
Non-service costs — Pension and OPEB (G)	(18)	0	(39)	(1)
Rolled Products [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Segment Adjusted EBITDA	78	185	251	325
Restructuring and other charges	(7)	(2)	(16)	(8)
Non-service costs — Pension and OPEB (G)	4	15	8	31
Provision for depreciation and amortization	(53)	(46)	(99)	(93)
Extrusions [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Segment Adjusted EBITDA	(13)	0	(1)	3
Restructuring and other charges	(3)	0	(28)	0
Non-service costs — Pension and OPEB (G)	2	5	3	9
Provision for depreciation and amortization	(6)	(8)	(12)	(15)
Building and Construction Systems [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Segment Adjusted EBITDA	(38)	38	(70)	65
Restructuring and other charges	(5)	(24)	(6)	(29)
Non-service costs — Pension and OPEB (G)	0	2	1	3
Provision for depreciation and amortization	(5)	(5)	(9)	(10)
Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Non-service costs — Pension and OPEB (G)	0	2	(1)	4
Corporate Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Corporate Expenses	(7)	(12)	(9)	(33)
Restructuring and other charges	(62)	(12)	(64)	(3)
Non-service costs — Pension and OPEB (G)	0	4	0	8
Operating segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Segment Adjusted EBITDA	103	223	320	393
Restructuring and other charges	(15)	(26)	(6)	(37)
Non-service costs — Pension and OPEB (G)	6	22	12	43
Provision for depreciation and amortization	(64)	(59)	(120)	(118)
Segment Reconciling Items [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Non-service costs — Pension and OPEB (G)	$ 0	$ 6	$ (1)	$ 12