Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive (Loss) Income by Component (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension and other postretirement benefits
Total Other comprehensive income (loss)	$ 31	$ (463)	$ 110	$ (363)
Foreign currency translation
Other comprehensive income	85	202	251	689
Available-for-sale securities
Other comprehensive (loss) income	1		(133)	4
Cash flow hedges
Total Other comprehensive income (loss)	10	(348)	13	(567)
Arconic [Member]
Pension and other postretirement benefits
Balance at beginning of period	(1,931)	(3,514)	(2,010)	(3,611)
Unrecognized net actuarial loss and prior service cost	(7)	(819)	4	(883)
Tax (expense) benefit	1	286	(3)	312
Total Other comprehensive income (loss) before reclassifications, net of tax	(6)	(533)	1	(571)
Amortization of net actuarial loss and prior service cost	56	109	167	317
Tax (expense) benefit	(19)	(38)	(58)	(111)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	37	71	109	206
Total Other comprehensive income (loss)	31	(462)	110	(365)
Balance at end of period	(1,900)	(3,976)	(1,900)	(3,976)
Foreign currency translation
Balance at beginning of period	(523)	(2,064)	(689)	(2,412)
Other comprehensive income	85	157	251	505
Balance at end of period	(438)	(1,907)	(438)	(1,907)
Available-for-sale securities
Balance at beginning of period	(2)	(1)	132	(5)
Other comprehensive (loss) income	1		(133)	4
Balance at end of period	(1)	(1)	(1)	(1)
Cash flow hedges
Balance at beginning of period	2	364	(1)	597
Net change from periodic revaluations	15	(430)	20	(772)
Tax (expense) benefit	(5)	126	(7)	229
Total Other comprehensive income (loss) before reclassifications, net of tax	10	(304)	13	(543)
Net amount reclassified to earnings		(46)		(41)
Tax benefit		12		13
Total amount reclassified from Accumulated other comprehensive loss, net of tax		(34)		(28)
Total Other comprehensive income (loss)	10	(338)	13	(571)
Balance at end of period	12	26	12	26
Noncontrolling Interests [Member]
Pension and other postretirement benefits
Balance at beginning of period		(53)		(56)
Unrecognized net actuarial loss and prior service cost		(1)
Total Other comprehensive income (loss) before reclassifications, net of tax		(1)
Amortization of net actuarial loss and prior service cost		1		3
Tax (expense) benefit		(1)		(1)
Total amount reclassified from Accumulated other comprehensive loss, net of tax				2
Total Other comprehensive income (loss)		(1)		2
Balance at end of period		(54)		(54)
Foreign currency translation
Balance at beginning of period	(2)	(641)	(2)	(780)
Other comprehensive income		45		184
Balance at end of period	$ (2)	(596)	$ (2)	(596)
Cash flow hedges
Balance at beginning of period		11		(3)
Net change from periodic revaluations		20		35
Tax (expense) benefit		(6)		(10)
Total Other comprehensive income (loss) before reclassifications, net of tax		14		25
Net amount reclassified to earnings		(34)		(29)
Tax benefit		10		8
Total amount reclassified from Accumulated other comprehensive loss, net of tax		(24)		(21)
Total Other comprehensive income (loss)		(10)		4
Balance at end of period		$ 1		$ 1