Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Apr. 01, 2019	Jan. 01, 2019	Apr. 01, 2018	Jan. 01, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 4,866	$ 5,218	$ 5,585	$ 4,924
Other comprehensive income:
Total Other comprehensive (loss) income, net of tax	(59)	12	0	100
Adoption of accounting standards (B)						$ 73
Balance at end of period	4,710	5,375	4,710	5,375
Arconic
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,869)	(2,556)	(2,926)	(2,644)
Other comprehensive income:
Total Other comprehensive (loss) income, net of tax	(59)	12		100
Adoption of accounting standards (B)						(2)
Balance at end of period	(2,928)	(2,544)	(2,928)	(2,544)
Pension and other postretirement benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,281)	(2,058)	(2,344)	(2,230)
Other comprehensive income:
Unrecognized net actuarial loss and prior service cost/benefit	(6)	30	60	152
Tax benefit (expense)	2	(7)	(13)	(35)
Total Other comprehensive (loss) income before reclassifications, net of tax	(4)	23	47	117
Amortization of net actuarial loss and prior service cost	39	18	54	118
Tax expense benefit	(9)	(4)	(12)	(26)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	30	14	42	92
Total Other comprehensive (loss) income, net of tax	26	37	89	209
Balance at end of period	(2,255)	(2,021)	(2,255)	(2,021)
Foreign currency translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(587)	(516)	(583)	(437)
Other comprehensive income:
Total Other comprehensive (loss) income, net of tax	(87)	(16)	(91)	(95)
Balance at end of period	(674)	(532)	(674)	(532)
Available-for-sale securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	(4)	(3)	(2)
Other comprehensive income:
Total Other comprehensive (loss) income, net of tax	1	1	4	(1)
Balance at end of period	1	(3)	1	(3)
Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(1)	22	4	25
Other comprehensive income:
Unrecognized net actuarial loss and prior service cost/benefit	0	(7)	(5)	(4)
Tax benefit (expense)	0	1	2	1
Total Other comprehensive (loss) income before reclassifications, net of tax	0	(6)	(3)	(3)
Amortization of net actuarial loss and prior service cost	2	(5)	1	(12)
Tax expense benefit	(1)	1	0	2
Total amount reclassified from Accumulated other comprehensive loss, net of tax	1	(4)	1	(10)
Total Other comprehensive (loss) income, net of tax	1	(10)	(2)	(13)
Balance at end of period	$ 0	$ 12	$ 0	$ 12
Accounting Standards Update 2017-12
Other comprehensive income:
Adoption of accounting standards (B)						2
Accounting Standards Update 2017-12 | Cash flow hedges
Other comprehensive income:
Adoption of accounting standards (B)					$ 0	$ (2)	$ 0	$ 0