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Significant accounting policies - Leases (Details)	12 Months Ended
	Dec. 31, 2019
Maximum
Disclosure of Finance Leases [Line Items]
Lease term	19 years

X

- Definition

[Line Items] for Disclosure of Finance Leases [Table]

+ References

No definition available.

+ Details

Name:	mx_DisclosureofFinanceLeasesLineItems
Namespace Prefix:	mx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Lease, Term

+ References

No definition available.

+ Details

Name:	mx_LeaseTerm
Namespace Prefix:	mx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	ifrs-full_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: