Debt (BNDES) - Additional Information (Detail)	12 Months Ended					12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2014 BRL	Dec. 31, 2013 USD ($)	Dec. 31, 2013 BRL	Jul. 25, 2014 USD ($)	Dec. 31, 2014 Aluminio [Member] USD ($)	Dec. 31, 2014 Aluminio [Member] BRL	Dec. 31, 2013 Aluminio [Member] USD ($)	Dec. 31, 2013 Aluminio [Member] BRL	Dec. 31, 2014 Loan Agreement with BNDES [Member] Agreement	Dec. 31, 2013 Loan Agreement with BNDES [Member]	Dec. 31, 2014 Three of the Subloans [Member] Loan Agreement with BNDES [Member] USD ($)	Dec. 31, 2014 Three of the Subloans [Member] Loan Agreement with BNDES [Member] BRL	Dec. 31, 2014 Two of the Subloans [Member] Loan Agreement with BNDES [Member] BRL	Dec. 31, 2014 Third Subloan [Member] Loan Agreement with BNDES [Member] BRL	Dec. 31, 2014 BNDES Loans, Due 2014-2029 [Member] USD ($)	Dec. 31, 2014 BNDES Loans, Due 2014-2029 [Member] BRL	Dec. 31, 2013 BNDES Loans, Due 2014-2029 [Member] USD ($)	Dec. 31, 2013 BNDES Loans, Due 2014-2029 [Member] BRL
Debt Instrument [Line Items]
Commitment on loan agreement	$ 85,000,000	177,000,000										$ 397,000,000	687,000,000
Long-term interest rate										5.00%	5.00%
Weighted-average margin on long term debt										1.48%	1.48%
Number of Subloan agreement										3
Principal and interest														667,000,000	20,000,000
Line of credit facility, outstanding borrowings					4,000,000,000			52,000,000								209,000,000	560,000,000	254,000,000	599,000,000
Weighted-average interest rate	6.49%	6.49%	6.49%	6.49%				0.72%	0.72%
Early repayment of outstanding borrowings	20,000,000	47,000,000	22,000,000	47,000,000		5,000,000	11,000,000	5,000,000	11,000,000
Additional borrowing amount	1,000,000	2,000,000	1,000,000	2,000,000
Interest rate margin	1.55%	1.55%
Outstanding borrowings	$ 58,000,000	156,000,000	$ 71,000,000	166,000,000
Interest rate of outstanding borrowings	6.55%	6.55%	6.55%	6.55%