Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 5,218	$ 5,753	$ 4,924	$ 5,141
Total Other comprehensive income, net of tax	12	127	100	241
Balance at end of period	5,375	5,961	5,375	5,961
Arconic
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,556)	(2,454)	(2,644)	(2,568)
Total Other comprehensive income, net of tax	12	127	100	241
Balance at end of period	(2,544)	(2,327)	(2,544)	(2,327)
Pension and other postretirement benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,058)	(1,931)	(2,230)	(2,010)
Other comprehensive income (loss), before reclassifications, before tax	30	(7)	152	4
Tax (expense) benefit	(7)	1	(35)	(3)
Total Other comprehensive income (loss) before reclassifications, net of tax	23	(6)	117	1
Reclassification from accumulated other comprehensive income, before tax	18	56	118	167
Tax (expense) benefit	(4)	(19)	(26)	(58)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	14	37	92	109
Total Other comprehensive income, net of tax	37	31	209	110
Balance at end of period	(2,021)	(1,900)	(2,021)	(1,900)
Foreign currency translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(516)	(523)	(437)	(689)
Total Other comprehensive income, net of tax	(16)	85	(95)	251
Balance at end of period	(532)	(438)	(532)	(438)
Available-for-sale securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(4)	(2)	(2)	132
Total Other comprehensive income, net of tax	1	1	(1)	(133)
Balance at end of period	(3)	(1)	(3)	(1)
Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	22	2	25	(1)
Other comprehensive income (loss), before reclassifications, before tax	(7)	15	(4)	20
Tax (expense) benefit	1	(5)	1	(7)
Total Other comprehensive income (loss) before reclassifications, net of tax	(6)	10	(3)	13
Reclassification from accumulated other comprehensive income, before tax	(5)	0	(12)	0
Tax (expense) benefit	1	0	2	0
Total amount reclassified from Accumulated other comprehensive loss, net of tax	(4)	0	(10)	0
Total Other comprehensive income, net of tax	(10)	10	(13)	13
Balance at end of period	12	12	12	12
Noncontrolling Interests
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at end of period	0	(2)	0	(2)
Pension and other postretirement benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	0	0	0
Other comprehensive income (loss), before reclassifications, before tax	0	0	0	0
Tax (expense) benefit	0	0	0	0
Total Other comprehensive income (loss) before reclassifications, net of tax	0	0	0	0
Reclassification from accumulated other comprehensive income, before tax	0	0	0	0
Tax (expense) benefit	0	0	0	0
Total amount reclassified from Accumulated other comprehensive loss, net of tax	0	0	0	0
Total Other comprehensive income, net of tax	0	0	0	0
Balance at end of period	0	0	0	0
Foreign currency translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	(2)	0	(2)
Total Other comprehensive income, net of tax	0	0	0	0
Balance at end of period	0	(2)	0	(2)
Available-for-sale securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	0	0	0
Total Other comprehensive income, net of tax	0	0	0	0
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	0	0	0	0
Other comprehensive income (loss), before reclassifications, before tax	0	0	0	0
Tax (expense) benefit	0	0	0	0
Total Other comprehensive income (loss) before reclassifications, net of tax	0	0	0	0
Reclassification from accumulated other comprehensive income, before tax	0	0	0	0
Tax (expense) benefit	0	0	0	0
Total amount reclassified from Accumulated other comprehensive loss, net of tax	0	0	0	0
Total Other comprehensive income, net of tax	0	0	0	0
Balance at end of period	$ 0	$ 0	$ 0	$ 0