Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period			$ 4,924	$ 5,141
Total Other comprehensive (loss) income, net of tax	$ (170)	$ 44	88	114
Balance at end of period	5,218	5,753	5,218	5,753
Arconic
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period			(2,644)	(2,568)
Total Other comprehensive (loss) income, net of tax			88	114
Balance at end of period	(2,556)	(2,454)	(2,556)	(2,454)
Pension and other postretirement benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,087)	(1,979)	(2,230)	(2,010)
Other comprehensive income (loss), before reclassifications, before tax	(15)	17	122	11
Tax benefit (expense)	3	(5)	(28)	(4)
Total Other comprehensive (loss) income before reclassifications, net of tax	(12)	12	94	7
Reclassification from accumulated other comprehensive income, before tax	52	56	100	111
Tax (expense) benefit	(11)	(20)	(22)	(39)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	41	36	78	72
Total Other comprehensive (loss) income, net of tax	29	48	172	79
Balance at end of period	(2,058)	(1,931)	(2,058)	(1,931)
Foreign currency translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(315)	(622)	(437)	(689)
Total Other comprehensive (loss) income, net of tax	(201)	99	(79)	166
Balance at end of period	(516)	(523)	(516)	(523)
Available-for-sale securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2)	99	(2)	132
Total Other comprehensive (loss) income, net of tax	(2)	(101)	(2)	(134)
Balance at end of period	(4)	(2)	(4)	(2)
Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	18	4	25	(1)
Other comprehensive income (loss), before reclassifications, before tax	9	(4)	3	4
Tax benefit (expense)	(1)	1	0	(2)
Total Other comprehensive (loss) income before reclassifications, net of tax	8	(3)	3	2
Reclassification from accumulated other comprehensive income, before tax	(4)	1	(7)	1
Tax (expense) benefit	0	0	1	0
Total amount reclassified from Accumulated other comprehensive loss, net of tax	(4)	1	(6)	1
Total Other comprehensive (loss) income, net of tax	4	(2)	(3)	3
Balance at end of period	22	2	22	2
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 benefit (expense)	0	0	0	0
Total Other comprehensive (loss) income 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 (loss) 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 (loss) 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 (loss) 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 benefit (expense)	0	0	0	0
Total Other comprehensive (loss) income 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 (loss) income, net of tax	0	0	0	0
Balance at end of period	$ 0	$ 0	$ 0	$ 0