Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 3,577
Other comprehensive income:
Total other comprehensive (loss) income, net of tax	$ (27)	$ 61	32	$ 30
Ending balance	3,579		3,579
Accumulated other comprehensive loss
Other comprehensive income:
Total other comprehensive (loss) income, net of tax	(27)	61	32	30
Ending balance	(1,911)	(1,929)	(1,911)	(1,929)
Pension and other postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(903)	(866)	(980)	(2,732)
Other comprehensive income:
Unrecognized net actuarial gain (loss) and prior service cost/benefit	1	(7)	68	(66)
Tax benefit (expense)	0	3	(15)	11
Total Other comprehensive income (loss) before reclassifications, net of tax	1	(4)	53	(55)
Net amount reclassified to earnings	15	16	46	133
Tax benefit (expense)	(3)	(4)	(9)	(24)
Amount reclassified from Accumulated other comprehensive loss, net of tax	12	12	37	109
Total other comprehensive (loss) income, net of tax	13	8	90	54
Transfer to Arconic Corporation	0	(22)	0	1,798
Ending balance	(890)	(880)	(890)	(880)
Foreign currency translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(992)	(1,097)	(966)	(596)
Other comprehensive income:
Total Other comprehensive income (loss) before reclassifications, net of tax	(36)	48	(62)	(39)
Amount reclassified from Accumulated other comprehensive loss, net of tax	0	0	0	14
Total other comprehensive (loss) income, net of tax	(36)	48	(62)	(25)
Transfer to Arconic Corporation	0	0	0	(428)
Ending balance	(1,028)	(1,049)	(1,028)	(1,049)
Cash flow hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	11	(5)	3	(1)
Other comprehensive income:
Unrecognized net actuarial gain (loss) and prior service cost/benefit	1	2	20	(6)
Tax benefit (expense)	0	2	(4)	2
Total Other comprehensive income (loss) before reclassifications, net of tax	1	4	16	(4)
Net amount reclassified to earnings	(7)	2	(15)	6
Tax benefit (expense)	2	(1)	3	(1)
Amount reclassified from Accumulated other comprehensive loss, net of tax	(5)	1	(12)	5
Total other comprehensive (loss) income, net of tax	(4)	5	4	1
Ending balance	$ 7	$ 0	$ 7	$ 0