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Post-Retirement Obligation- Summary of Independent Actuaries Estimate for the Expense Amount (Detail) - BRL (R$) R$ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of defined benefit plans [line items] |  |  |
Cost of current service | R$ 19 | R$ 17 | R$ 17
Interest on the actuarial obligation | 1,246 | 1,263 | 1,153
Total expense in 2018 as per actuarial opinion | 390 | 781 | 545
Pension plans and retirement supplement plans [member] |  |  |
Disclosure of defined benefit plans [line items] |  |  |
Total expense in 2018 as per actuarial opinion | 374 | 274 | 535
Health plan [member] |  |  |
Disclosure of defined benefit plans [line items] |  |  |
Cost of current service | 11 | 9 | 7
Interest on the actuarial obligation | 178 | 173 | 135
Total expense in 2018 as per actuarial opinion | 22 | 305 | 138
Dental plan [member] |  |  |
Disclosure of defined benefit plans [line items] |  |  |
Cost of current service |  |  | 1
Interest on the actuarial obligation | 3 | 4 | 3
Total expense in 2018 as per actuarial opinion | (1) | 6 | (4)
Life insurance [member] |  |  |
Disclosure of defined benefit plans [line items] |  |  |
Total expense in 2018 as per actuarial opinion | (5) | R$ 196 | R$ (124)
2017 business year [Member] |  |  |
Disclosure of defined benefit plans [line items] |  |  |
Cost of current service | 15 |  |
Interest on the actuarial obligation | 1,161 |  |
Expected return on the assets of the Plan | (771) |  |
Total expense in 2018 as per actuarial opinion | 405 |  |
2017 business year [Member] | Pension plans and retirement supplement plans [member] |  |  |
Disclosure of defined benefit plans [line items] |  |  |
Cost of current service | 3 |  |
Interest on the actuarial obligation | 959 |  |
Expected return on the assets of the Plan | (771) |  |
Total expense in 2018 as per actuarial opinion | 191 |  |
2017 business year [Member] | Health plan [member] |  |  |
Disclosure of defined benefit plans [line items] |  |  |
Cost of current service | 10 |  |
Interest on the actuarial obligation | 173 |  |
Total expense in 2018 as per actuarial opinion | 183 |  |
2017 business year [Member] | Dental plan [member] |  |  |
Disclosure of defined benefit plans [line items] |  |  |
Interest on the actuarial obligation | 4 |  |
Total expense in 2018 as per actuarial opinion | 4 |  |
2017 business year [Member] | Life insurance [member] |  |  |
Disclosure of defined benefit plans [line items] |  |  |
Cost of current service | 2 |  |
Interest on the actuarial obligation | 25 |  |
Total expense in 2018 as per actuarial opinion | R$ 27 |  |

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- Definition The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in financial assumptions that result in remeasurements of the net defined benefit liability (asset). Financial assumptions deal with items such as: (a) the discount rate; (b) benefit levels, excluding any cost of the benefits to be met by employees, and future salary; (c) in the case of medical benefits, future medical costs, including claim handling costs (ie the costs that will be incurred in processing and resolving claims, including legal and adjuster's fees); and (d) taxes payable by the plan on contributions relating to service before the reporting date or on benefits resulting from that service. [Refer: Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2017-01-01 -Paragraph 141 -Subparagraph c -Clause iii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_c_iii&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2017-01-01 -Paragraph 141 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_a&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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- Definition The increase (decrease) in the net defined benefit liability (asset) resulting from the passage of time. [Refer: Interest expense; Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2017-01-01 -Paragraph 141 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_b&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets. The return on plans assets is interest, dividends and other revenue derived from the plan assets, together with realised and unrealised gains or losses on the plan assets, less any costs of managing plan assets and less any tax payable by the plan itself, other than tax included in the actuarial assumptions used to measure the present value of the defined benefit obligation. [Refer: Plan assets [member]; Net defined benefit liability (asset); Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2017-01-01 -Paragraph 141 -Subparagraph c -Clause i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_c_i&doctype=Standard -URIDate 2017-03-09 + Details
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