Employee benefits (Schedule of Components of Expense) (Details) - MXN ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of defined benefit plans [abstract]
Cost of sales	$ 19,494	$ 23,296	$ (5,550)
Administrative expenses	15,316	18,302	(4,360)
Net cost of benefits	$ 34,810	$ 41,598	$ (9,910)

Administrative expenses during period for defined benefit plan.

No definition available.

Name:	simecb_DefinedBenefitAdministrativeExpenses
Namespace Prefix:	simecb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration