Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Jan. 01, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 5,585	$ 4,924
Other comprehensive income:
Total Other comprehensive income, net of tax	76	258
Adoption of accounting standards (B)			$ 73
Balance at end of period	5,178	5,282
Arconic
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,926)	(2,644)
Other comprehensive income:
Total Other comprehensive income, net of tax	76	258
Adoption of accounting standards (B)			(2)
Balance at end of period	(2,852)	(2,386)
Pension and other postretirement benefits (F)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,344)	(2,230)
Other comprehensive income:
Unrecognized net actuarial loss and prior service cost/benefit	72	137
Tax (expense) benefit	(16)	(31)
Total Other comprehensive income before reclassifications, net of tax	56	106
Amortization of net actuarial loss and prior service cost	(21)	48
Tax benefit (expense)	5	(11)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	(16)	37
Total Other comprehensive income, net of tax	40	143
Balance at end of period	(2,304)	(2,087)
Foreign currency translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(583)	(437)
Other comprehensive income:
Total Other comprehensive income, net of tax	26	122
Balance at end of period	(557)	(315)
Available-for-sale securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(3)	(2)
Other comprehensive income:
Total Other comprehensive income, net of tax	3	0
Balance at end of period	0	(2)
Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	4	25
Other comprehensive income:
Unrecognized net actuarial loss and prior service cost/benefit	8	(6)
Tax (expense) benefit	(1)	1
Total Other comprehensive income before reclassifications, net of tax	7	(5)
Amortization of net actuarial loss and prior service cost	0	(3)
Tax benefit (expense)	0	1
Total amount reclassified from Accumulated other comprehensive loss, net of tax	0	(2)
Total Other comprehensive income, net of tax	7	(7)
Balance at end of period	$ 9	$ 18
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)			$ (2)	$ 0