Post-Retirement Obligation - Disclosure of Defined Benefit Plans Expense Recognised in Income Statement (Detail) - BRL (R$) R$ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of fair value of plan assets [line items]
Cost of current service	R$ 19	R$ 17	R$ 17
Interest on the actuarial obligation	1,246	1,263	1,153
Expected return on the assets of the Plan	(810)	(833)	(933)
Past service cost	(619)		(74)
Expense according to actuarial calculation	(164)	447	285
Expense as per actuarial opinion [member]
Disclosure of fair value of plan assets [line items]
Expense according to actuarial calculation			163
Adjustment relating to debt [member]
Disclosure of fair value of plan assets [line items]
Expense according to actuarial calculation			122
Pension plans and retirement supplement plans [member]
Disclosure of fair value of plan assets [line items]
Cost of current service	5	5	6
Interest on the actuarial obligation	980	1,014	934
Expected return on the assets of the Plan	(810)	(833)	(933)
Expense according to actuarial calculation	175	186	129
Pension plans and retirement supplement plans [member] \| Expense as per actuarial opinion [member]
Disclosure of fair value of plan assets [line items]
Expense according to actuarial calculation			7
Pension plans and retirement supplement plans [member] \| Adjustment relating to debt [member]
Disclosure of fair value of plan assets [line items]
Expense according to actuarial calculation			122
Health plan [member]
Disclosure of fair value of plan assets [line items]
Cost of current service	11	9	7
Interest on the actuarial obligation	178	173	135
Expense according to actuarial calculation	189	182	142
Health plan [member] \| Expense as per actuarial opinion [member]
Disclosure of fair value of plan assets [line items]
Expense according to actuarial calculation			142
Dental plan [member]
Disclosure of fair value of plan assets [line items]
Cost of current service			1
Interest on the actuarial obligation	3	4	3
Expense according to actuarial calculation	3	4	4
Dental plan [member] \| Expense as per actuarial opinion [member]
Disclosure of fair value of plan assets [line items]
Expense according to actuarial calculation			4
Life insurance [member]
Disclosure of fair value of plan assets [line items]
Cost of current service	3	3	3
Interest on the actuarial obligation	85	72	81
Past service cost	(619)		(74)
Expense according to actuarial calculation	R$ (531)	R$ 75	10
Life insurance [member] \| Expense as per actuarial opinion [member]
Disclosure of fair value of plan assets [line items]
Expense according to actuarial calculation			R$ 10

The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)]

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

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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.

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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

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from past service cost. Past service cost is the change in the present value of the defined benefit obligation for employee service in prior periods, resulting from a plan amendment (the introduction or withdrawal of, or changes to, a defined benefit plan) or a curtailment (a significant reduction by the entity in the number of employees covered by a plan). [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_d&doctype=Standard
-URIDate 2017-03-09

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- Definition

Expected return on plan assets net defined benefit liability asset.

+ References

No definition available.

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- Definition

Expense net defined benefit liability asset.

+ References

No definition available.

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