Restructuring and Other Charges - Additional Information (Detail)		3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 11, 2017 Potline Mg	Sep. 30, 2017 USD ($) Employees	Sep. 30, 2016 USD ($) Employees	Sep. 30, 2017 USD ($) Employees	Sep. 30, 2016 USD ($) Employees	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges		$ (10,000,000)	$ 17,000,000	$ 12,000,000	$ 109,000,000
Number of employees associated with layoff costs | Employees		10		120
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		$ 29,000,000
Asset retirement obligations		20,000,000
Environmental remediation obligations		4,000,000
Severance costs		5,000,000
Carrying value of the smelter and related assets		17,254,000,000		$ 17,254,000,000		$ 16,741,000,000
Amount of cash payments expected to be paid beyond the end of the current annual period				15,000,000
Pension Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges		3,000,000		6,000,000
Smelting and Related Assets [Member]
Restructuring Cost and Reserve [Line Items]
Carrying value of the smelter and related assets							$ 0	$ 0
Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Inventory write down					$ 5,000,000
Warrick Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges				$ 84,000,000
Aluminum Segment [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employees			30	110	60
Restructuring Programs Layoffs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges		4,000,000	$ 17,000,000	$ 17,000,000	$ 28,000,000
Corporate Restructuring Charges [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			1,000,000		1,000,000
Miscellaneous Charges [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges		2,000,000	1,000,000	2,000,000
Asset Impairment and Accelerated Depreciation [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					70,000,000
Severance costs			4,000,000		24,000,000
Other costs			2,000,000		38,000,000
Contract Termination [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges		6,000,000		24,000,000
Contract Termination [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Other costs			3,000,000		30,000,000
Asset Retirement Obligations [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges						$ 97,000,000
Asset Retirement Obligations [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Other costs			3,000,000		4,000,000
Other Costs [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Other costs			$ 2,000,000		4,000,000
Other Adjustments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges		29,000,000		38,000,000	$ 4,000,000
Employee Relocation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges		$ 7,000,000		$ 7,000,000
Restructuring Programs Layoffs 2017 [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employees				120
Approximate number of employees already laid off | Employees		70		70
Cash payments made against the layoff reserves		$ 2,000,000		$ 7,000,000
Restructuring Programs Layoffs 2016 [Member] | Maximum [Member]
Restructuring Cost and Reserve [Line Items]
Cash payments made against the layoff reserves		1,000,000		2,000,000
Restructuring Programs Layoffs 2015 [Member]
Restructuring Cost and Reserve [Line Items]
Cash payments made against the layoff reserves		$ 4,000,000		$ 16,000,000