Expiry date 2019-01-01: The amount of property (land or a building - or part of a building - or both) held (by the owner or by the lessee under a finance lease) to earn rentals or for capital appreciation or both, rather than for: (a) use in the production or supply of goods or services or for administrative purposes; or (b) sale in the ordinary course of business. Effective 2019-01-01: The amount of property (land or a building - or part of a building - or both) held (by the owner or by the lessee as a right-of-use asset) to earn rentals or for capital appreciation or both, rather than for: (a) use in the production or supply of goods or services or for administrative purposes; or (b) sale in the ordinary course of business.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 40
-IssueDate 2017-01-01
-Paragraph 76
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=40&date=2017-03-01&anchor=para_76&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 40
-IssueDate 2017-01-01
-Paragraph 79
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=40&date=2017-03-01&anchor=para_79_d&doctype=Standard
-URIDate 2017-03-09

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 54
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_54_b&doctype=Standard
-URIDate 2017-03-09

Name:	ifrs-full_InvestmentProperty
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Property, Plant and Equipment - Summary of Accumulated Investments in Consortium (Detail) - BRL (R$) R$ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure of detailed information about property, plant and equipment [line items]
Investment Property	R$ 618	R$ 801	R$ 861	R$ 906
Average annual depreciation rate	3.14%	3.51%	3.39%
Cemig GT [member]
Disclosure of detailed information about property, plant and equipment [line items]
Investment Property	R$ 126	R$ 127
Cemig GT [member] \| Electricity Generation Consortium [member] \| Investment Property Completed [member] \| Gross Carrying Amount [member] \| Queimado plant [member]
Disclosure of detailed information about property, plant and equipment [line items]
Investment Property	R$ 217	R$ 217
Stake in energy generated	82.50%
Average annual depreciation rate		4.00%
Cemig GT [member] \| Electricity Generation Consortium [member] \| Investment Property Completed [member] \| Gross Carrying Amount [member] \| Accumulated Depreciation and Amortisation [member]
Disclosure of detailed information about property, plant and equipment [line items]
Investment Property	R$ (91)	R$ (90)