Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive (Loss) Income by Component (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension and other postretirement benefits
Balance at beginning of period	$ (2,644)
Total Other comprehensive income	143	$ 31
Balance at end of period	(2,386)
Foreign currency translation
Other comprehensive income	122	67
Available-for-sale securities
Other comprehensive (loss) income	0	(33)
Cash flow hedges
Total Other comprehensive income (loss)	(7)	5
Arconic
Pension and other postretirement benefits
Balance at beginning of period	(2,230)	(2,010)
Unrecognized net actuarial loss and prior service cost/benefit	137	(6)
Tax (expense) benefit	31	(1)
Total Other comprehensive income (loss) before reclassifications, net of tax	106	(5)
Amortization of net actuarial loss and prior service cost	48	55
Tax expense	(11)	(19)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	37	36
Total Other comprehensive income	143	31
Balance at end of period	(2,087)	(1,979)
Foreign currency translation
Balance at beginning of period	(437)	(689)
Other comprehensive income	122	67
Balance at end of period	(315)	(622)
Available-for-sale securities
Balance at beginning of period	(2)	132
Other comprehensive (loss) income	0	(33)
Balance at end of period	(2)	99
Cash flow hedges
Balance at beginning of period	25	(1)
Net change from periodic revaluations	(6)	8
Tax (expense) benefit	1	(3)
Total Other comprehensive income (loss) before reclassifications, net of tax	(5)	5
Net amount reclassified to earnings	(3)	0
Tax benefit	1	0
Total amount reclassified from Accumulated other comprehensive loss, net of tax	(2)	0
Total Other comprehensive income (loss)	(7)	5
Balance at end of period	18	4
Noncontrolling Interests
Pension and other postretirement benefits
Balance at beginning of period	0	0
Unrecognized net actuarial loss and prior service cost/benefit	0	0
Tax (expense) benefit	0	0
Total Other comprehensive income (loss) before reclassifications, net of tax	0	0
Amortization of net actuarial loss and prior service cost	0	0
Tax expense	0	0
Total amount reclassified from Accumulated other comprehensive loss, net of tax	0	0
Total Other comprehensive income	0	0
Balance at end of period	0	0
Foreign currency translation
Balance at beginning of period	0	(2)
Other comprehensive income	0	0
Balance at end of period	0	(2)
Available-for-sale securities
Balance at beginning of period	0	0
Other comprehensive (loss) income	0	0
Balance at end of period	0	0
Cash flow hedges
Balance at beginning of period	0	0
Net change from periodic revaluations	0	0
Tax (expense) benefit	0	0
Total Other comprehensive income (loss) before reclassifications, net of tax	0	0
Net amount reclassified to earnings	0	0
Tax benefit	0	0
Total amount reclassified from Accumulated other comprehensive loss, net of tax	0	0
Total Other comprehensive income (loss)	0	0
Balance at end of period	$ 0	$ 0