Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 4,605	$ 5,569	$ 4,908
Other comprehensive (loss) income:
Total Other comprehensive income (loss), net of tax	16	(401)	85
Ending balance	3,577	4,605	5,569
Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		73	0
Other comprehensive (loss) income:
Ending balance			73
Accumulated other comprehensive loss balance at end of period
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(3,329)	(2,926)	(2,644)
Other comprehensive (loss) income:
Total Other comprehensive income (loss), net of tax	16	(401)	85
Ending balance	(1,943)	(3,329)	(2,926)
Accumulated other comprehensive loss balance at end of period | Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(2)	(367)
Other comprehensive (loss) income:
Ending balance			(2)
Pension and other postretirement benefits (H)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(2,732)	(2,344)	(2,230)
Other comprehensive (loss) income:
Unrecognized net actuarial (loss) gain and prior service cost/benefit	(211)	(587)	70
Tax benefit (expense)	48	129	(19)
Total Other comprehensive (loss) income before reclassifications, net of tax	(163)	(458)	51
Amortization of net actuarial loss and prior service cost	149	90	262
Tax expense	(32)	(20)	(58)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	117	70	204
Total Other comprehensive income (loss), net of tax	(46)	(388)	255
Transfer to Arconic Corporation	1,798	0	0
Ending balance	(980)	(2,732)	(2,344)
Pension and other postretirement benefits (H) | Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	0	0	(369)
Other comprehensive (loss) income:
Ending balance		0	0
Foreign currency translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(596)	(583)	(437)
Other comprehensive (loss) income:
Total Other comprehensive (loss) income before reclassifications, net of tax	58	(13)	(146)
Transfer to Arconic Corporation	(428)	0	0
Ending balance	(966)	(596)	(583)
Debt securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	0	(3)	(2)
Other comprehensive (loss) income:
Total Other comprehensive income (loss), net of tax	0	3	(1)
Ending balance	0	0	(3)
Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(1)	4	25
Other comprehensive (loss) income:
Unrecognized net actuarial (loss) gain and prior service cost/benefit	0	(9)	(15)
Tax benefit (expense)	0	3	3
Total Other comprehensive (loss) income before reclassifications, net of tax	0	(6)	(12)
Amortization of net actuarial loss and prior service cost	6	4	(14)
Tax expense	(2)	(1)	3
Total amount reclassified from Accumulated other comprehensive loss, net of tax	4	3	(11)
Total Other comprehensive income (loss), net of tax	4	(3)	(23)
Ending balance	3	(1)	4
Cash flow hedges | Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 0	(2)	2
Other comprehensive (loss) income:
Ending balance		$ 0	$ (2)