Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Jan. 01, 2020	Jan. 01, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 4,621	$ 5,585
Other comprehensive income:
Total Other comprehensive (loss) income, net of tax	(40)	76
Foreign currency translation adjustments	(65)	26
Adoption of accounting standards				$ 73
Balance at end of period	4,797	5,178
Arconic
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(3,329)	(2,926)
Other comprehensive income:
Total Other comprehensive (loss) income, net of tax	(40)	76
Adoption of accounting standards				(2)
Balance at end of period	(3,369)	(2,852)
Pension and other postretirement benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,732)	(2,344)
Other comprehensive income:
Unrecognized net actuarial loss and prior service cost/benefit	1	72
Tax expense	0	(16)
Total Other comprehensive income before reclassifications, net of tax	1	56
Amortization of net actuarial loss and prior service cost	43	(21)
Tax (expense) benefit	(7)	5
Amount reclassified from Accumulated other comprehensive loss, net of tax	36	(16)
Total Other comprehensive (loss) income, net of tax	37	40
Balance at end of period	(2,695)	(2,304)
Foreign currency translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(596)	(583)
Other comprehensive income:
Amount reclassified from Accumulated other comprehensive loss, net of tax	14	0
Total Other comprehensive (loss) income, net of tax	(65)	26
Foreign currency translation adjustments	(79)	26
Balance at end of period	(661)	(557)
Available-for-sale securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	(3)
Other comprehensive income:
Total Other comprehensive (loss) income, net of tax	1	3
Balance at end of period	1	0
Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(1)	4
Other comprehensive income:
Unrecognized net actuarial loss and prior service cost/benefit	(11)	8
Tax expense	(1)	(1)
Total Other comprehensive income before reclassifications, net of tax	(12)	7
Amortization of net actuarial loss and prior service cost	(1)	0
Tax (expense) benefit	0	0
Amount reclassified from Accumulated other comprehensive loss, net of tax	(1)	0
Total Other comprehensive (loss) income, net of tax	(13)	7
Balance at end of period	$ (14)	$ 9
Accounting Standards Update 2017-12 | Cash flow hedges
Other comprehensive income:
Adoption of accounting standards			$ 0	$ (2)