Restructuring and Other Charges (2017 Actions) - Additional Information (Detail) $ in Millions	1 Months Ended		3 Months Ended								9 Months Ended	12 Months Ended
	Oct. 31, 2017 USD ($) a	Jul. 31, 2017 USD ($) Potline kt	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) a Employee kt	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			$ 138				$ 297					$ 527	$ 309	$ 318
Cash payment												102	73	109
Sales			3,344	$ 3,390	$ 3,579	$ 3,090	3,174	$ 2,964	$ 2,859	$ 2,655		13,403	11,652	9,318
Asset impairments												18	40	155
Other costs												48	49	44
Asset retirement obligations												5	10	97
Environmental remediation obligations												2	8	26
Severance costs												2	15	15
Carrying value of the smelter and related assets			$ 15,938				17,447					15,938	17,447
Smelting and Related Assets [Member]
Restructuring Cost and Reserve [Line Items]
Carrying value of the smelter and related assets							$ 0						0
Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												312	253	176
2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges													$ 309
Number of employees associated with layoff costs | Employee													130
Other costs													$ 16
Inventory write down													$ 6
Number of potlines restarted | Potline		3
Capacity of restarted potlines | kt		161
Total number of potlines | Potline		5
Total capacity of potlines | kt		269
Liabilities related to original closure decision reversed		$ 33
Asset retirement obligations		20
Environmental remediation obligations		4
Severance costs		$ 9
2017 Restructuring Plans Action [Member] | Smelting and Related Assets [Member]
Restructuring Cost and Reserve [Line Items]
Carrying value of the smelter and related assets															$ 0	$ 0
2017 Restructuring Plans Action [Member] | Aluminum Segment [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employee													115
Rarly Termination of Power Contract [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges													$ 244
Exit Cost [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges													49
Contract Termination [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges													$ 41
Cash payment	$ 238
Area of land transferred | a	2,200
Net asset carrying value	$ 6
Sales											$ 105			141
Cost of goods sold											$ 148			210
Capacity closure | kt													191
Area of land owned | a													30,000
Asset impairments													$ 32
Cash payments made against the layoff reserves													1
Other costs													$ 16
Contract Termination [Member] | 2017 Restructuring Plans Action [Member] | Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employee													10
Restructuring Programs Layoffs 2017 [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges													$ 22
Cash payments made against the layoff reserves												3	9
Reserve Previously Established [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges													22
Other Item Charges [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges													18
Other Adjustments [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges													43
Asset Retirement Obligations [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												5	10	97
Asset Retirement Obligations [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Other costs													8
Environmental Remediation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												$ 2	$ 8	$ 26