Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 3,577
Other comprehensive income:
Total Other comprehensive income (loss), net of tax	$ 57	$ 9	59	$ (31)
Ending balance	3,598		3,598
Accumulated other comprehensive loss
Other comprehensive income:
Total Other comprehensive income (loss), net of tax	57	9	59	(31)
Ending balance	(1,884)	(1,968)	(1,884)	(1,968)
Pension and other postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(938)	(2,695)	(980)	(2,732)
Other comprehensive income:
Unrecognized net actuarial gain (loss) and prior service cost/benefit	30	(60)	67	(59)
Tax (expense) benefit	(7)	8	(15)	8
Total Other comprehensive income (loss) before reclassifications, net of tax	23	(52)	52	(51)
Net amount reclassified to earnings	15	74	31	117
Tax (expense) benefit	(3)	(13)	(6)	(20)
Amount reclassified from Accumulated other comprehensive loss, net of tax	12	61	25	97
Total Other comprehensive income (loss), net of tax	35	9	77	46
Transfer to Arconic Corporation	0	1,820	0	1,820
Ending balance	(903)	(866)	(903)	(866)
Foreign currency translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,010)	(661)	(966)	(596)
Other comprehensive income:
Total Other comprehensive income (loss) before reclassifications, net of tax	18	(8)	(26)	(87)
Amount reclassified from Accumulated other comprehensive loss, net of tax	0	0	0	14
Total Other comprehensive income (loss), net of tax	18	(8)	(26)	(73)
Transfer to Arconic Corporation	0	(428)	0	(428)
Ending balance	(992)	(1,097)	(992)	(1,097)
Debt securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0	1	0	0
Other comprehensive income:
Total Other comprehensive income (loss), net of tax	0	(1)	0	0
Ending balance	0	0	0	0
Cash flow hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	7	(14)	3	(1)
Other comprehensive income:
Unrecognized net actuarial gain (loss) and prior service cost/benefit	11	3	19	(8)
Tax (expense) benefit	(2)	1	(4)	0
Total Other comprehensive income (loss) before reclassifications, net of tax	9	4	15	(8)
Net amount reclassified to earnings	(5)	5	(8)	4
Tax (expense) benefit	0	0	1	0
Amount reclassified from Accumulated other comprehensive loss, net of tax	(5)	5	(7)	4
Total Other comprehensive income (loss), net of tax	4	9	8	(4)
Ending balance	$ 11	$ (5)	$ 11	$ (5)