Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		$ 4,797	$ 5,178	$ 4,621	$ 5,585
Other comprehensive income:
Foreign currency translation adjustments		(8)	(30)	(73)	(4)
Total Other comprehensive income (loss), net of tax		9	(17)	(31)	59
Balance at end of period		3,449	4,866	3,449	4,866
Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	[1]				73
Accumulated other comprehensive loss
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(3,369)	(2,852)	(3,329)	(2,926)
Other comprehensive income:
Total Other comprehensive income (loss), net of tax		9	(17)	(31)	59
Balance at end of period		(1,968)	(2,869)	(1,968)	(2,869)
Accumulated other comprehensive loss | Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	[1]				(2)
Pension and other postretirement benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(2,695)	(2,304)	(2,732)	(2,344)
Other comprehensive income:
Unrecognized net actuarial loss and prior service cost/benefit		(60)	(6)	(59)	66
Tax expense		8	1	8	(15)
Total Other comprehensive loss (income) before reclassifications, net of tax		(52)	(5)	(51)	51
Amortization of net actuarial loss and prior service cost		74	36	117	15
Tax (expense) benefit		(13)	(8)	(20)	(3)
Amount reclassified from Accumulated other comprehensive loss, net of tax		61	28	97	12
Total Other comprehensive income (loss), net of tax		9	23	46	63
Transfer to Arconic Corporation		1,820	0	1,820	0
Balance at end of period		(866)	(2,281)	(866)	(2,281)
Foreign currency translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(661)	(557)	(596)	(583)
Other comprehensive income:
Foreign currency translation adjustments		(8)	(30)	(87)	(4)
Amount reclassified from Accumulated other comprehensive loss, net of tax		0	0	14	0
Total Other comprehensive income (loss), net of tax		(8)	(30)	(73)	(4)
Transfer to Arconic Corporation		(428)	0	(428)	0
Balance at end of period		(1,097)	(587)	(1,097)	(587)
Available-for-sale securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		1	0	0	(3)
Other comprehensive income:
Total Other comprehensive income (loss), net of tax		(1)	0	0	3
Balance at end of period		0	0	0	0
Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(14)	9	(1)	4
Other comprehensive income:
Unrecognized net actuarial loss and prior service cost/benefit		3	(13)	(8)	(5)
Tax expense		1	3	0	2
Total Other comprehensive loss (income) before reclassifications, net of tax		4	(10)	(8)	(3)
Amortization of net actuarial loss and prior service cost		5	(1)	4	(1)
Tax (expense) benefit		0	1	0	1
Amount reclassified from Accumulated other comprehensive loss, net of tax		5	0	4	0
Total Other comprehensive income (loss), net of tax		9	(10)	(4)	(3)
Balance at end of period		(5)	(1)	(5)	(1)
Cash flow hedges | Cumulative Effect, Period of Adoption, Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		$ 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.