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Securities - Schedule of Cash Investments (Parenthetical) (Detail) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Bottom of range [member] | Financial Notes-Banks [member] |  |
Disclosure of investment securities [Line Items] |  |
Investment rate remunerated at percentage of rate for interbank deposits | 102.01% | 104.25%
Bottom of range [member] | Debentures [member] |  |
Disclosure of investment securities [Line Items] |  |
Investment rate remunerated at percentage of rate for interbank deposits | 104.25% | 104.25%
Top of range [member] | Financial Notes-Banks [member] |  |
Disclosure of investment securities [Line Items] |  |
Investment rate remunerated at percentage of rate for interbank deposits | 112.00% | 112.70%
Top of range [member] | Debentures [member] |  |
Disclosure of investment securities [Line Items] |  |
Investment rate remunerated at percentage of rate for interbank deposits | 161.54% | 113.00%
Bank certificates of deposit [member] | Bottom of range [member] | Fixed And Floating Interest Rate [member] |  |
Disclosure of investment securities [Line Items] |  |
Investment rate remunerated at percentage of rate for interbank deposits | 100.25% | 98.50%
Bank certificates of deposit [member] | Top of range [member] | Fixed And Floating Interest Rate [member] |  |
Disclosure of investment securities [Line Items] |  |
Investment rate remunerated at percentage of rate for interbank deposits | 105.25% | 111.00%

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