Restructuring and Other Charges (2016 Actions) - Additional Information (Detail) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) Employee kt
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	$ 138	$ 297	$ 527	$ 309	$ 318
Severance costs			2	15	15
Other costs			48	49	44
Asset impairments			18	40	155
2016 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					$ 318
Number of employees associated with layoff costs | Employee					75
2016 Restructuring Plans Action [Member] | AofA [Member]
Restructuring Cost and Reserve [Line Items]
Percentage of ownership in gas exploration assets					43.00%
Impaired interest amount					$ 72
2016 Restructuring Plans Action [Member] | Warrick Smelter, Wenatchee Smelter and Point Comfort [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					87
2016 Restructuring Plans Action [Member] | Western Australia [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					$ 72
2016 Restructuring Plans Action [Member] | Suriname [Member]
Restructuring Cost and Reserve [Line Items]
Capacity closure | kt					2,207
2016 Restructuring Plans Action [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Other costs					$ 156
Inventory write down					5
Other related costs					$ 4
2016 Restructuring Plans Action [Member] | Aluminum Segment [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employee					60
2016 Restructuring Plans Action [Member] | Bauxite [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employee					15
Exit Cost [Member] | 2016 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					$ 131
Restructuring Programs Layoffs 2016 [Member] | 2016 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					32
Cash payments made against the layoff reserves				2	7
Other Item Charges [Member] | 2016 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					8
Other Adjustments [Member] | 2016 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					12
Asset Impairment and Accelerated Depreciation [Member] | 2016 Restructuring Plans Action [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					70
Severance costs					24
Other costs					156
Shutdown and Curtailment Actions [Member] | 2016 Restructuring Plans Action [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Asset impairments					16
Asset Retirement Obligations [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			5	10	97
Asset Retirement Obligations [Member] | 2016 Restructuring Plans Action [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Other costs					94
Environmental Remediation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			$ 2	$ 8	26
Environmental Remediation [Member] | 2016 Restructuring Plans Action [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Other costs					26
Contract Termination [Member] | 2016 Restructuring Plans Action [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Other costs					$ 32