Employee benefits, Net cost for period (Details) - MXN ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Details for net cost [Abstract]
Interest cost	$ (257)	$ (208)
Net cost for the period	19,027	22,873
Pensions and Seniority Premiums [Member]
Details for net cost [Abstract]
Current service cost	4,541	4,709
Interest cost	11,892	13,643
Net cost for the period	16,433	18,352
Termination of Employment [Member]
Details for net cost [Abstract]
Current service cost	1,727	2,178
Interest cost	867	2,343
Net cost for the period	$ 2,594	$ 4,521

No definition available.

Name:	ifrs-full_ChangesInNetDefinedBenefitLiabilityAssetAbstract
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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 2020-01-01
-Paragraph 141
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_a&doctype=Standard
-URIDate 2020-01-01

+ Details

Name:	ifrs-full_CurrentServiceCostNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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 2020-01-01
-Paragraph 141
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141&doctype=Standard
-URIDate 2020-01-01

+ Details

Name:	ifrs-full_IncreaseDecreaseInNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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 2020-01-01
-Paragraph 141
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_b&doctype=Standard
-URIDate 2020-01-01

+ Details

Name:	ifrs-full_InterestExpenseIncomeNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	ifrs-full_DefinedBenefitPlansAxis=gtmay_PensionsAndSeniorityPremiumsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_DefinedBenefitPlansAxis=gtmay_TerminationOfEmploymentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: