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EMPLOYEE BENEFIT PLANS (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
EMPLOYEE BENEFIT PLANS |  |  |
Period of completion of services for qualification of defined contribution plan | 90 days | 90 days | 90 days
Defined contribution plan, percentage of employer matching contribution | 50.00% | 50.00% | 50.00%
Defined contribution plan, percentage of participant contributions | 5.00% | 5.00% | 5.00%
Employee contributions vesting period | 5 years | 5 years | 5 years
Defined contribution plan, employer contribution | $ 1,030 | $ 917 | $ 833

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- Definition Represents employee contributions vesting period. + References No definition available. + Details
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Namespace Prefix: | mlr_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents period of completion of services for qualification of defined contribution plan. + References No definition available. + Details
Name: | mlr_DefinedContributionPlanNumberOfCompletedPeriodOfServicesForQualificationForPlan
Namespace Prefix: | mlr_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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

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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage employer matches of the employee's percentage contribution matched. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration