Debt (Credit Facilities) - Additional Information (Detail) - USD ($)		12 Months Ended
	Jul. 25, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2016
Debt Instrument [Line Items]
Ratio of indebtedness to consolidated EBITDA		5.50%
Ratio of indebtedness to consolidated EBITDA, after December 31, 2019		1.00%
Extension of maturity period		1 year
Debt instrument, extended maturity date		Jul. 25, 2020
Outstanding borrowings		$ 715,000,000
Line of credit facility, borrowing		$ 1,950,000,000	$ 1,890,000,000	$ 1,640,000,000
Weighted-average maturity days		49 days	69 days	67 days
Arconic [Member]
Debt Instrument [Line Items]
Line of credit facility, repayment		$ 1,950,000,000	$ 1,890,000,000	$ 1,640,000,000
Weighted-average interest rate		1.88%	1.61%	1.54%
Maximum [Member]
Debt Instrument [Line Items]
Applicable margin on LIBOR loans		1.70%
Minimum [Member]
Debt Instrument [Line Items]
Applicable margin on LIBOR loans		0.70%
Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Revolving credit facility, term	5 years
Revolving Credit Agreement [Member] | Prior to Amendment [Member]
Debt Instrument [Line Items]
Line of credit facility, outstanding borrowings					$ 4,000,000,000
Revolving Credit Agreement [Member] | After Amendment [Member]
Debt Instrument [Line Items]
Line of credit facility, outstanding borrowings					$ 3,000,000,000
Commercial Paper [Member] | Maximum [Member]
Debt Instrument [Line Items]
Fees paid to maintain credit facility		0.30%
Senior Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, outstanding borrowings		$ 0	$ 0
Amounts borrowed under the credit agreement		0	$ 0	$ 0
Expected Maturity Date Year 2017 [Member]
Debt Instrument [Line Items]
Outstanding borrowings		$ 465,000,000
Line of credit facility, expiration date		Dec. 31, 2017
Expected Maturity Date Year 2018 [Member]
Debt Instrument [Line Items]
Outstanding borrowings		$ 250,000,000
Line of credit facility, expiration date		Dec. 31, 2018