Restructuring and Other Charges (2017 Actions) - Additional Information (Detail)	1 Months Ended		3 Months Ended								4 Months Ended	12 Months Ended
	Oct. 31, 2017 USD ($) a	Jul. 31, 2017 USD ($) Potline Mg	Dec. 31, 2017 USD ($) Employees	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Employees	Dec. 31, 2017 USD ($) a Employees kt	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			$ 297,000,000				$ 209,000,000					$ 309,000,000	$ 318,000,000	$ 983,000,000
Cash payment												73,000,000	109,000,000	66,000,000
Sales			3,174,000,000	$ 2,964,000,000	$ 2,859,000,000	$ 2,655,000,000	2,537,000,000	$ 2,329,000,000	$ 2,323,000,000	$ 2,129,000,000		$ 11,652,000,000	9,318,000,000	11,199,000,000
Capacity closure | kt												191
Asset impairments												$ 40,000,000	155,000,000	311,000,000
Other costs												49,000,000	47,000,000	92,000,000
Asset retirement obligations												10,000,000	97,000,000	76,000,000
Environmental remediation obligations												8,000,000	26,000,000	86,000,000
Severance costs												23,000,000	32,000,000	199,000,000
Carrying value of the smelter and related assets			17,447,000,000				$ 16,741,000,000				$ 17,447,000,000	17,447,000,000	16,741,000,000
Smelting and Related Assets [Member]
Restructuring Cost and Reserve [Line Items]
Carrying value of the smelter and related assets			$ 0								0	0
Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												253,000,000	176,000,000	773,000,000
2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												$ 309,000,000
Number of employees associated with layoff costs | Employees												130
Inventory write down												$ 6,000,000
Total number of potlines | Potline		5
Total metric tons capacity of potlines | Mg		268,800
Number of potlines restarted | Potline		3
Metric tons capacity of restarted potlines | Mg		161,400
Liabilities related to original closure decision reversed												$ 33,000,000
Asset retirement obligations		$ 20,000,000
Environmental remediation obligations		4,000,000
Severance costs		$ 9,000,000
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 | Employees												115
Contract Termination [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												$ 41,000,000
Cash payment	$ 238,000,000
Net asset carrying value	$ 6,000,000
Area of land transferred | a	2,200
Sales											105,000,000		141,000,000	147,000,000
Cost of goods sold											$ 148,000,000		210,000,000	201,000,000
Area of land owned | a												30,000
Capacity closure | kt												191
Asset impairments												$ 32,000,000
Cash payments made against the layoff reserves												1,000,000
Other costs												$ 16,000,000
Contract Termination [Member] | 2017 Restructuring Plans Action [Member] | Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employees												10
Asset Retirement Obligations [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												$ 10,000,000	97,000,000	76,000,000
Asset Retirement Obligations [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Other costs												8,000,000
Environmental Remediation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												8,000,000	$ 26,000,000	$ 86,000,000
Environmental Remediation [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Other costs												$ 16,000,000
Restructuring Programs Layoffs 2017 [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employees												140
Cash payment												$ 9,000,000
Approximate number of employees already laid off | Employees			90								90	90
Rarly Termination of Power Contract [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												$ 244,000,000
Exit Cost [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												49,000,000
Restructuring Programs Layoffs [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												22,000,000
Reserve Previously Established [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												22,000,000
Miscellaneous Charges [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												18,000,000
Other Adjustments [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												$ 43,000,000