Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		$ 3,449	$ 4,850	$ 4,605	$ 5,569
Other comprehensive income:
Total Other comprehensive income (loss), net of tax		61	(59)	30	0
Ending balance		3,501	4,694	3,501	4,694
Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	[1]				73
Accumulated other comprehensive loss
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(1,968)	(2,869)	(3,329)	(2,926)
Other comprehensive income:
Total Other comprehensive income (loss), net of tax		61	(59)	30
Ending balance		(1,929)	(2,928)	(1,929)	(2,928)
Accumulated other comprehensive loss | Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	[1]				(2)
Pension and other postretirement benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(866)	(2,281)	(2,732)	(2,344)
Other comprehensive income:
Unrecognized net actuarial loss and prior service cost/benefit		(7)	(6)	(66)	60
Tax expense		3	2	11	(13)
Total Other comprehensive income (loss) before reclassifications, net of tax		(4)	(4)	(55)	47
Amortization of net actuarial loss and prior service cost		16	39	133	54
Tax (expense) benefit		(4)	(9)	(24)	(12)
Amount reclassified from Accumulated other comprehensive loss, net of tax		12	30	109	42
Total Other comprehensive income (loss), net of tax		8	26	54	89
Transfer to Arconic Corporation		(22)	0	1,798	0
Ending balance		(880)	(2,255)	(880)	(2,255)
Foreign currency translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(1,097)	(587)	(596)	(583)
Other comprehensive income:
Total Other comprehensive income (loss) before reclassifications, net of tax		48	(87)	(39)	(91)
Amount reclassified from Accumulated other comprehensive loss, net of tax		0	0	14	0
Total Other comprehensive income (loss), net of tax		48	(87)	(25)	(91)
Transfer to Arconic Corporation		0	0	(428)	0
Ending balance		(1,049)	(674)	(1,049)	(674)
Available-for-sale securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		0	0	0	(3)
Other comprehensive income:
Total Other comprehensive income (loss), net of tax		0	1	0	4
Ending balance		0	1	0	1
Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(5)	(1)	(1)	4
Other comprehensive income:
Unrecognized net actuarial loss and prior service cost/benefit		2	0	(6)	(5)
Tax expense		2	0	2	2
Total Other comprehensive income (loss) before reclassifications, net of tax		4	0	(4)	(3)
Amortization of net actuarial loss and prior service cost		2	2	6	1
Tax (expense) benefit		(1)	(1)	(1)	0
Amount reclassified from Accumulated other comprehensive loss, net of tax		1	1	5	1
Total Other comprehensive income (loss), net of tax		5	1	1	(2)
Ending balance		0	0	0	0
Cash flow hedges | Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		$ 0	$ 0	$ 0	$ (2)

[1] The Company entered into a sale leaseback arrangement in October 2018 for a cast house that is now part of Arconic Corporation, and due to continuing involvement, the gain on sale was deferred. In connection with the adoption of the new lease accounting standard on January 1, 2019, the arrangement no longer required that the gain be deferred. As such, the associated $73 deferred gain, net of tax was recognized as a cumulative effect of an accounting change within Accumulated deficit. Also, the Company adopted the new hedge accounting guidance on January 1, 2019. As a result, an adjustment of $2 was recognized as a cumulative effect of an accounting change within Accumulated deficit with an offset to Accumulated other comprehensive loss related to the elimination of a separate measurement of ineffectiveness for its cash flow hedges.