Pension and Other Postretirement Benefits - Schedule of Obligations and Funded Status (Detail) - USD ($) $ in Millions		12 Months Ended
	Apr. 18, 2017	Dec. 31, 2017	Dec. 31, 2016
Change in plan assets
Employer contributions	$ 50
Pension benefits
Change in benefit obligation
Benefit obligation at beginning of year		$ 7,026	$ 14,247
Service cost		90	165
Interest cost		234	435
Amendments		1	2
Actuarial (gains) losses		311	770
Transfer to Alcoa Corporation		0	(7,577)
Settlements		0	(82)
Benefits paid, net of participants’ contributions		(425)	(794)
Medicare Part D subsidy receipts		0	0
Foreign currency translation impact		122	(140)
Benefit obligation at end of year		7,359	7,026
Change in plan assets
Fair value of plan assets at beginning of year		4,666	10,928
Actual return on plan assets		212	89
Employer contributions		310	296
Participants’ contributions		0	16
Benefits paid		(404)	(762)
Administrative expenses		(33)	(65)
Transfer to Alcoa Corporation		0	(5,610)
Settlements		0	(82)
Foreign currency translation impact		111	(144)
Fair value of plan assets at end of year		4,862	4,666
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status		(2,497)	(2,360)
Less: Amounts attributed to joint venture partners		0	0
Net funded status		(2,497)	(2,360)
Amounts recognized in the Consolidated Balance Sheet consist of:
Noncurrent assets		89	6
Current liabilities		(22)	(21)
Noncurrent liabilities		(2,564)	(2,345)
Net amount recognized		(2,497)	(2,360)
Amounts recognized in Accumulated Other Comprehensive Loss consist of:
Net actuarial loss		3,240	2,979
Prior service cost (benefit)		10	15
Total, before tax effect		3,250	2,994
Less: Amounts attributed to joint venture partners		0	0
Net amount recognized, before tax effect		3,250	2,994
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Loss consist of:
Net actuarial loss (gain)		481	(1,992)
Amortization of accumulated net actuarial loss		(220)	(380)
Prior service (benefit) cost		0	(42)
Amortization of prior service (cost) benefit		(5)	(13)
Total, before tax effect		256	(2,427)
Less: Amounts attributed to joint venture partners		0	38
Net amount recognized, before tax effect		256	(2,389)
Other postretirement benefits
Change in benefit obligation
Benefit obligation at beginning of year		980	2,319
Service cost		7	13
Interest cost		30	63
Amendments		0	0
Actuarial (gains) losses		1	112
Transfer to Alcoa Corporation		0	(1,340)
Settlements		0	0
Benefits paid, net of participants’ contributions		(98)	(197)
Medicare Part D subsidy receipts		7	9
Foreign currency translation impact		0	1
Benefit obligation at end of year		927	980
Change in plan assets
Fair value of plan assets at beginning of year		0	0
Actual return on plan assets		0	0
Employer contributions		0	0
Participants’ contributions		0	0
Benefits paid		0	0
Administrative expenses		0	0
Transfer to Alcoa Corporation		0	0
Settlements		0	0
Foreign currency translation impact		0	0
Fair value of plan assets at end of year		0	0
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status		(927)	(980)
Less: Amounts attributed to joint venture partners		0	0
Net funded status		(927)	(980)
Amounts recognized in the Consolidated Balance Sheet consist of:
Noncurrent assets		0	0
Current liabilities		(86)	(91)
Noncurrent liabilities		(841)	(889)
Net amount recognized		(927)	(980)
Amounts recognized in Accumulated Other Comprehensive Loss consist of:
Net actuarial loss		146	150
Prior service cost (benefit)		(37)	(45)
Total, before tax effect		109	105
Less: Amounts attributed to joint venture partners		0	0
Net amount recognized, before tax effect		109	105
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Loss consist of:
Net actuarial loss (gain)		1	(224)
Amortization of accumulated net actuarial loss		(5)	(24)
Prior service (benefit) cost		0	37
Amortization of prior service (cost) benefit		8	24
Total, before tax effect		4	(187)
Less: Amounts attributed to joint venture partners		0	0
Net amount recognized, before tax effect		4	(187)
United States | Pension benefits
Change in benefit obligation
Benefit obligation at beginning of year		5,707
Benefit obligation at end of year		6,018	5,707
Change in plan assets
Fair value of plan assets at beginning of year		3,495
Fair value of plan assets at end of year		3,544	3,495
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status		$ (2,474)	$ (2,212)