Restructuring and Other Charges - Restructuring Reserve (Details) $ in Millions	3 Months Ended			6 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($) employee	Mar. 31, 2020 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($) employee	Sep. 30, 2021 USD ($) employee	Sep. 30, 2020 USD ($)
Restructuring Reserve [Roll Forward]
Beginning balance			$ 21		$ 14	$ 14	$ 21
Restructuring Reserve, Accrual Adjustment			2
Cash payments			(27)			(11)
Restructuring charges			27			7
Other			(9)			(4)
Ending balance	$ 6					6
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business	(14)	$ (3)				(612)	(61)
2021 Restructuring Programs
Restructuring Reserve [Roll Forward]
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business	(14)					$ (612)
2020 Restructuring Programs
Restructuring Reserve [Roll Forward]
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business		$ (3)					(61)
Number of employees separated | employee						485
Expected number of employees separated | employee		450			550	500
Extrusions | 2021 Restructuring Programs
Restructuring Reserve [Roll Forward]
Number of employees separated | employee						115
Operating segments
Restructuring Reserve [Roll Forward]
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business	(3)	$ 0				$ (6)	6
Operating segments | Rolled Products
Restructuring Reserve [Roll Forward]
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business	0	1				1	(15)
Operating segments | Building and Construction Systems
Restructuring Reserve [Roll Forward]
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business	0	0				1	(6)
Operating segments | Extrusions
Restructuring Reserve [Roll Forward]
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business	(3)	1				(6)	(27)
Corporate, Non-Segment
Restructuring Reserve [Roll Forward]
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business	(11)	(3)				(606)	(67)
Other costs
Restructuring Reserve [Roll Forward]
Beginning balance			1		$ 1	1	1
Restructuring Reserve, Accrual Adjustment			0
Cash payments			(3)			(2)
Restructuring charges			4			5
Other			(1)			(1)
Ending balance	3					3
Other costs | 2021 Restructuring Programs
Restructuring Reserve [Roll Forward]
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business	(1)					(2)
Other costs | 2020 Restructuring Programs
Restructuring Reserve [Roll Forward]
Restructuring, Settlement And Impairment Provisions, Including Gain (Loss) On Disposition Of Assets And Business		$ (1)					(7)
Layoff costs
Restructuring Reserve [Roll Forward]
Beginning balance			20		$ 13	13	$ 20
Restructuring Reserve, Accrual Adjustment			2
Cash payments			(24)			(9)
Restructuring charges			23			2
Other			$ (8)			(3)
Ending balance	3					3
Layoff costs | 2021 Restructuring Programs
Restructuring Reserve [Roll Forward]
Cash payments	(1)					(2)
Layoff costs | 2020 Restructuring Programs
Restructuring Reserve [Roll Forward]
Cash payments	(1)					(4)
Layoff costs | 2019 Restructuring Program [Member]
Restructuring Reserve [Roll Forward]
Cash payments	$ (1)					$ (3)
Special Termination Benefits | Forecast
Restructuring Reserve [Roll Forward]
Cash payments				$ (2)