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Financial Instruments and Risk Management - Summary of Credit Exposure (Detail) - Credit Risk [Member]
 12 Months Ended
Dec. 31, 2017
BRL (R$)
A1 [Member]  
Disclosure of credit risk exposure [line items]  
Bank equity R$ 3,500,000,000
Concentration Minimum of 80%
A2 Rating [Member]  
Disclosure of credit risk exposure [line items]  
Concentration Maximum 20%
B [Member]  
Disclosure of credit risk exposure [line items]  
Concentration Maximum 20%
Bottom of range [member] | A1 [Member]  
Disclosure of credit risk exposure [line items]  
Percentage of limit per bank entity 6.00%
Bottom of range [member] | A2 Rating [Member]  
Disclosure of credit risk exposure [line items]  
Bank equity R$ 1,000,000,000
Percentage of limit per bank entity 5.00%
Bottom of range [member] | B [Member]  
Disclosure of credit risk exposure [line items]  
Bank equity R$ 400,000,000,000
Percentage of limit per bank entity 5.00%
Top of range [member] | A1 [Member]  
Disclosure of credit risk exposure [line items]  
Percentage of limit per bank entity 9.00%
Top of range [member] | A2 Rating [Member]  
Disclosure of credit risk exposure [line items]  
Bank equity R$ 3,500,000,000
Percentage of limit per bank entity 8.00%
Top of range [member] | B [Member]  
Disclosure of credit risk exposure [line items]  
Bank equity R$ 1,000,000,000
Percentage of limit per bank entity 7.00%