XML 99 R80.htm IDEA: XBRL DOCUMENT
v3.20.4

Benefit Plans, Investment Strategies (Details) | 12 Months Ended
--- | ---
Dec. 31, 2020
Defined Benefit Plan, Plan Assets, Allocations [Abstract] |
Average rate of return on assets | 9.00%
Equity Securities [Member] | Minimum [Member] |
Defined Benefit Plan, Plan Assets, Allocations [Abstract] |
Target long-term investment portfolio mix | 55.00%
Equity Securities [Member] | Maximum [Member] |
Defined Benefit Plan, Plan Assets, Allocations [Abstract] |
Target long-term investment portfolio mix | 65.00%
Fixed Income Securities [Member] | Minimum [Member] |
Defined Benefit Plan, Plan Assets, Allocations [Abstract] |
Target long-term investment portfolio mix | 30.00%
Fixed Income Securities [Member] | Maximum [Member] |
Defined Benefit Plan, Plan Assets, Allocations [Abstract] |
Target long-term investment portfolio mix | 35.00%
Other Securities [Member] | Minimum [Member] |
Defined Benefit Plan, Plan Assets, Allocations [Abstract] |
Target long-term investment portfolio mix | 0.00%
Other Securities [Member] | Maximum [Member] |
Defined Benefit Plan, Plan Assets, Allocations [Abstract] |
Target long-term investment portfolio mix | 15.00%

X
- Definition The weighted average return on plan assets calculated based on the average number of years specified. + References No definition available. + Details
Name: | mtx_AverageRateOfReturnOnAssets
Namespace Prefix: | mtx_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanAssetsTargetAllocationsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_DefinedBenefitPlanEquitySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_FixedIncomeSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_OtherDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |