Debt (BNDES) - Additional Information (Detail) BRL in Millions, $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2014 USD ($)	Dec. 31, 2015 USD ($) Agreement	Dec. 31, 2015 BRL Agreement	Dec. 31, 2014 USD ($)	Dec. 31, 2014 BRL	Dec. 31, 2015 BRL	Dec. 31, 2014 BRL	Jul. 25, 2014 USD ($)
Debt Instrument [Line Items]
Commitment on loan agreement		$ 85				BRL 177
Line of credit facility, outstanding borrowings | $								$ 4,000
Weighted-average interest rate		8.49%	8.49%	8.49%	8.49%
Early repayment of outstanding borrowings		$ 15	BRL 48	$ 20	BRL 47
Additional borrowing amount | $	$ 1,238
Interest rate margin		1.55%	1.55%
Outstanding borrowings		$ 38		$ 58		BRL 146	BRL 156
Interest rate of outstanding borrowings		6.55%	6.55%	6.55%	6.55%
Loan Agreement with BNDES [Member]
Debt Instrument [Line Items]
Long-term interest rate		7.00%		5.00%		7.00%	5.00%
Weighted-average margin on long term debt		1.48%	1.48%	1.48%	1.48%
Number of Subloan agreement | Agreement		3	3
Aluminio [Member]
Debt Instrument [Line Items]
Early repayment of outstanding borrowings		$ 3	BRL 10	$ 5	BRL 11
Three of the Subloans [Member] | Loan Agreement with BNDES [Member]
Debt Instrument [Line Items]
Commitment on loan agreement		397				BRL 687
Two of the Subloans [Member] | Loan Agreement with BNDES [Member]
Debt Instrument [Line Items]
Principal and interest | BRL						667
Third Subloan [Member] | Loan Agreement with BNDES [Member]
Debt Instrument [Line Items]
Principal and interest | BRL						20
BNDES Loans, Due 2014-2029 [Member]
Debt Instrument [Line Items]
Line of credit facility, outstanding borrowings		136		209		BRL 522	BRL 560
Additional borrowing amount		$ 1	BRL 1	$ 1	BRL 2