Intangible Assets - Gross Carrying Amount and Accumulated Amortization (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 187,571	$ 217,380
Accumulated Amortization	(128,190)	(119,944)
Impairment		(28,794)
Net Carrying Amount	59,381	68,642
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Amount	112,109	116,101
Accumulated Amortization	(80,319)	(75,649)
Impairment		(2,206)
Net Carrying Amount	$ 31,790	38,246
Customer relationships \| Minimum
Finite-Lived Intangible Assets [Line Items]
Useful Life (Years)	5 years
Customer relationships \| Maximum
Finite-Lived Intangible Assets [Line Items]
Useful Life (Years)	18 years
Software/Technology
Finite-Lived Intangible Assets [Line Items]
Useful Life (Years)	3 years
Gross Amount	$ 52,265	77,326
Accumulated Amortization	(26,415)	(23,519)
Impairment		(25,874)
Net Carrying Amount	$ 25,850	27,933
Software/Technology \| Minimum
Finite-Lived Intangible Assets [Line Items]
Useful Life (Years)	3 years
Software/Technology \| Maximum
Finite-Lived Intangible Assets [Line Items]
Useful Life (Years)	15 years
Covenants not to compete
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 12,623	12,833
Accumulated Amortization	(12,390)	(12,162)
Impairment		(212)
Net Carrying Amount	$ 233	459
Covenants not to compete \| Minimum
Finite-Lived Intangible Assets [Line Items]
Useful Life (Years)	2 years
Covenants not to compete \| Maximum
Finite-Lived Intangible Assets [Line Items]
Useful Life (Years)	5 years
Other
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 10,574	11,120
Accumulated Amortization	(9,066)	(8,614)
Impairment		(502)
Net Carrying Amount	$ 1,508	$ 2,004
Other \| Minimum
Finite-Lived Intangible Assets [Line Items]
Useful Life (Years)	2 years
Other \| Maximum
Finite-Lived Intangible Assets [Line Items]
Useful Life (Years)	12 years

Finite-Lived Intangible Assets, Accumulated Impairment

No definition available.

Name:	mg_FiniteLivedIntangibleAssetsAccumulatedImpairment
Namespace Prefix:	mg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 928
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6473545&loc=d3e61844-108004

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

Name:	us-gaap_FiniteLivedIntangibleAssetsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph ((a)(1),(b))
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16212-109274

+ Details

Name:	us-gaap_IntangibleAssetsNetExcludingGoodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CustomerRelationshipsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_ComputerSoftwareIntangibleAssetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_NoncompeteAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_OtherIntangibleAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: