Accrued Severance Benefits - Additional Information (Detail)	9 Months Ended
Sep. 30, 2020
Percentage of employees eligible for severance benefits	98.00%
Korea [Member] \| Maximum [Member]
Retirement age of employees	60 years

Employees eligible for severance benefits percentage.

No definition available.

Name:	mx_EmployeesEligibleForSeveranceBenefitsPercentage
Namespace Prefix:	mx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant