Subsequent Events - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended	0 Months Ended
	Jun. 12, 2012 Installment	Mar. 31, 2015	Apr. 08, 2015 Installment Refinery
Subsequent Event [Line Items]
Number of installments	5
Alcoa Of Australia Limited [Member] | Majority-Owned Subsidiary | Service Agreements [Member]
Subsequent Event [Line Items]
Gas supply agreement period		12 years
Subsequent Event [Member] | Alcoa Of Australia Limited [Member] | Majority-Owned Subsidiary | Service Agreements [Member]
Subsequent Event [Line Items]
Number of alumina refineries to be powered under supplied agreement			3
Number of installments			2
Contract termination penalty			$ 0
First installment paid at the time of third party obligation			500
Subsequent Event [Member] | Alcoa Of Australia Limited [Member] | Majority-Owned Subsidiary | Service Agreements [Member] | First Installment [Member]
Subsequent Event [Line Items]
First installment paid at the time of third party obligation			300
Subsequent Event [Member] | Alcoa Of Australia Limited [Member] | Majority-Owned Subsidiary | Service Agreements [Member] | Second Installment [Member]
Subsequent Event [Line Items]
First installment paid at the time of third party obligation			$ 200