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v3.21.1

Defined Contribution Plan - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 5 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | May 22, 2020 | Dec. 31, 2020
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Defined contribution plan, description | 401(k) Savings Plan |  |  |
Defined contribution plan, maximum annual contributions per employee, percent | 85.00% |  |  | 85.00%
Defined contribution plan, employer matching contribution, percent of match |  |  | 100.00% |
Defined contribution plan participant's compensation percent |  |  | 3.00% |
Defined contribution plan percentage of participant's elective deferrals |  |  | 50.00% |
Selling, General and Administrative Expenses |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Defined contribution plan, employer discretionary contribution amount | $ 0.0 | $ 0.8 |  |
Minimum |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Defined contribution plan participant's compensation percent | 3.00% |  |  |
Maximum |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Defined contribution plan participant's compensation percent | 4.00% |  |  |

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- Definition Defined contribution plan participant's compensation percent. + References No definition available. + Details
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Namespace Prefix: | meg_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Defined contribution plan percentage of participants elective deferrals. + References No definition available. + Details
Name: | meg_DefinedContributionPlanPercentageOfParticipantsElectiveDeferrals
Namespace Prefix: | meg_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + 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 employer matches of the employee's percentage contribution matched. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Description of defined contribution pension and other postretirement plans. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
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- Details
Name: | us-gaap_IncomeStatementLocationAxis=us-gaap_SellingGeneralAndAdministrativeExpensesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |