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Post-Retirement Obligation - Disclosure of Changes In Present Value of Defined Benefit Obligation (Detail) - BRL (R$) R$ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of net defined benefit liability (asset) [line items]
Cost of current service	R$ (19)	R$ (17)	R$ (17)
Interest on the actuarial obligation	(1,246)	(1,263)	(1,153)
Actuarial losses (gains):
Cost of current service	(19)	(17)	(17)
Interest on the actuarial obligation	(1,246)	(1,263)	(1,153)
Due to changes in financial assumptions	(390)	(781)	(545)
Pension plans and retirement supplement plans [member]
Disclosure of net defined benefit liability (asset) [line items]
Cost of current service	(5)	(5)	(6)
Interest on the actuarial obligation	(980)	(1,014)	(934)
Actuarial losses (gains):
Cost of current service	(5)	(5)	(6)
Interest on the actuarial obligation	(980)	(1,014)	(934)
Benefits paid	(841)	(807)	(734)
Health plan [member]
Disclosure of net defined benefit liability (asset) [line items]
Cost of current service	(11)	(9)	(7)
Interest on the actuarial obligation	(178)	(173)	(135)
Actuarial losses (gains):
Cost of current service	(11)	(9)	(7)
Interest on the actuarial obligation	(178)	(173)	(135)
Due to changes in financial assumptions	(22)	(305)	(138)
Dental plan [member]
Disclosure of net defined benefit liability (asset) [line items]
Cost of current service			(1)
Interest on the actuarial obligation	(3)	(4)	(3)
Actuarial losses (gains):
Cost of current service			(1)
Interest on the actuarial obligation	(3)	(4)	(3)
Due to changes in financial assumptions	1	(6)	4
Life insurance [member]
Disclosure of net defined benefit liability (asset) [line items]
Cost of current service	(3)	(3)	(3)
Interest on the actuarial obligation	(85)	(72)	(81)
Actuarial losses (gains):
Cost of current service	(3)	(3)	(3)
Interest on the actuarial obligation	(85)	(72)	(81)
Present value of defined benefit obligation [member]
Disclosure of net defined benefit liability (asset) [line items]
Beginning balance	12,306	9,956	9,956
Cost of current service	19	17	17
Interest on the actuarial obligation	1,246	1,263	1,153
Actuarial losses (gains):
Cost of current service	19	17	17
Interest on the actuarial obligation	1,246	1,263	1,153
Due to changes in demographic assumptions	191	(1)	(19)
Due to changes in financial assumptions	537	1,828	(817)
Due to adjustments based on experience	(53)	162	565
Due to changes and adjustments	675	1,989	(271)
Plan amendment - Past service	(619)		(74)
Benefits paid	(964)	(919)	(825)
Ending balance	12,663	12,306	9,956
Present value of defined benefit obligation [member] | Pension plans and retirement supplement plans [member]
Disclosure of net defined benefit liability (asset) [line items]
Beginning balance	9,743	8,049	8,124
Cost of current service	5	5	6
Interest on the actuarial obligation	980	1,013	934
Actuarial losses (gains):
Cost of current service	5	5	6
Interest on the actuarial obligation	980	1,013	934
Due to changes in demographic assumptions	191	(1)	8
Due to changes in financial assumptions	414	1,253	(822)
Due to adjustments based on experience	53	231	533
Due to changes and adjustments	658	1,483	(281)
Benefits paid	(841)	(807)	(734)
Ending balance	10,545	9,743	8,049
Present value of defined benefit obligation [member] | Health plan [member]
Disclosure of net defined benefit liability (asset) [line items]
Beginning balance	1,711	1,323	1,120
Cost of current service	11	9	7
Interest on the actuarial obligation	178	174	135
Actuarial losses (gains):
Cost of current service	11	9	7
Interest on the actuarial obligation	178	174	135
Due to changes in demographic assumptions			43
Due to changes in financial assumptions	66	391	128
Due to adjustments based on experience	(44)	(87)	(33)
Due to changes and adjustments	22	304	138
Benefits paid	(113)	(99)	(77)
Ending balance	1,809	1,711	1,323
Present value of defined benefit obligation [member] | Dental plan [member]
Disclosure of net defined benefit liability (asset) [line items]
Beginning balance	38	30	32
Cost of current service			1
Interest on the actuarial obligation	3	4	3
Actuarial losses (gains):
Cost of current service			1
Interest on the actuarial obligation	3	4	3
Due to changes in demographic assumptions			1
Due to changes in financial assumptions	2	9	(1)
Due to adjustments based on experience	(2)	(3)	(4)
Due to changes and adjustments		6	(4)
Benefits paid	(2)	(2)	(2)
Ending balance	39	38	30
Present value of defined benefit obligation [member] | Life insurance [member]
Disclosure of net defined benefit liability (asset) [line items]
Beginning balance	814	554	680
Cost of current service	3	3	3
Interest on the actuarial obligation	85	72	81
Actuarial losses (gains):
Cost of current service	3	3	3
Interest on the actuarial obligation	85	72	81
Due to changes in demographic assumptions			(71)
Due to changes in financial assumptions	55	175	(122)
Due to adjustments based on experience	(60)	21	69
Due to changes and adjustments	(5)	196	(124)
Plan amendment - Past service	(619)		(74)
Benefits paid	(8)	(11)	(12)
Ending balance	R$ 270	R$ 814	R$ 554

X

- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in demographic assumptions that result in remeasurements of the net defined benefit liability (asset). Demographic assumptions deal with matters such as: (a) mortality; (b) rates of employee turnover, disability and early retirement; (c) the proportion of plan members with dependants who will be eligible for benefits; (d) the proportion of plan members who will select each form of payment option available under the plan terms; and (e) claim rates under medical plans. [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 ii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_c_ii&doctype=Standard
-URIDate 2017-03-09

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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
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-Paragraph 141
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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
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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.

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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from settlements. Settlements are transactions that eliminate all further legal or constructive obligations for part or all of the benefits provided under a defined benefit plan, other than a payment of benefits to, or on behalf of, employees that is set out in the terms of the plan and included in the actuarial assumptions. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]; Actuarial assumptions [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
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-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_d&doctype=Standard
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- Definition

The increase (decrease) in the net defined benefit liability (asset). [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
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141&doctype=Standard
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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
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- Definition

The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 140
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

+ References

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-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
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Actuarial gains (losses) arising from adjustments based on experience net defined benefit liability (asset).

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