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Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Commitments And Contingencies [Line Items] |  |
Lessee operating lease, expiration year | 2030 |
Rent expense | $ 2.5 | $ 2.1
Office Equipment |  |
Commitments And Contingencies [Line Items] |  |
Lessee operating lease, expiration year | 2025 |

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- Definition Commitments and contingencies. + References No definition available. + Details
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Namespace Prefix: | meg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Lessee operating lease, expiration year. + References No definition available. + Details
Name: | meg_LesseeOperatingLeaseExpirationYear
Namespace Prefix: | meg_
Data Type: | xbrli:gYearItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of operating lease expense. Excludes sublease income. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121603541&loc=SL77918638-209977 + Details
Name: | us-gaap_OperatingLeaseExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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