Income Taxes - Reconciliation of U.S. Federal Statutory Rate to Alcoa's Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
U.S. federal statutory rate	35.00%	35.00%	35.00%
Taxes on foreign operations	(3.60%)	(0.30%)	(0.10%)
Permanent differences on restructuring and other charges and asset disposals	(1.70%)	(0.80%)	10.80%
Audit and other adjustments to prior years' accruals		(0.90%)	3.50%
Noncontrolling interests	6.80%	(3.10%)	3.80%
Statutory tax rate and law changes	17.90%	0.60%	(0.40%)
Tax holidays	7.10%
Changes in valuation allowances	6.60%	(23.20%)	15.20%
Impairment of goodwill		(33.30%)
Amortization of goodwill related to intercompany stock sales/reorganizations	(0.10%)	1.10%	(7.70%)
Change in legal structure of investments			(4.10%)
Interest income related to income tax positions	(0.50%)		(1.30%)
Company-owned life insurance/split-dollar net premiums	(2.20%)	1.10%	(3.90%)
Other	(0.90%)	0.20%	(0.80%)
Effective tax rate	64.40%	(23.60%)	50.00%