Aggregate value used to determine number of shares underlying the warrant and issuable upon exercise of warrants.

No definition available.

Name:	meg_AggregateValueUsedToDetermineNumberOfSharesUnderlyingTheWarrantAndIssuableUponExerciseOfWarrants
Namespace Prefix:	meg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair Value Of Warrants

+ References

No definition available.

+ Details

Name:	meg_FairValueOfWarrants
Namespace Prefix:	meg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Exercise price per share or per unit of warrants or rights outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

+ Details

Name:	us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
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_ClassOfWarrantOrRightLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares.

+ References

+ Details

Name:	us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of new shares issued in the conversion of stock in a noncash (or part noncash) transaction. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=121583591&loc=d3e4332-108586

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121583591&loc=d3e4313-108586

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=121583591&loc=d3e4304-108586

+ Details

Name:	us-gaap_ConversionOfStockSharesIssued1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of expense (income) related to adjustment to fair value of warrant liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section 25
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=109262497&loc=d3e20148-110875

+ Details

Name:	us-gaap_FairValueAdjustmentOfWarrants
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

No definition available.

+ Details

Name:	us-gaap_SharePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Period between issuance and expiration of outstanding warrant and right embodying unconditional obligation requiring redemption by transferring asset at specified or determinable date or upon event certain to occur, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_WarrantsAndRightsOutstandingTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_RedeemablePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_OtherNonoperatingIncomeExpenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=meg_RedeemableSeriesAonePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=meg_RedeemableSeriesA2PreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=meg_ConvertibleAndRedeemableSeriesAtwoPreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Warrant Options - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 27, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jul. 30, 2020	Apr. 30, 2020	Apr. 13, 2020	Dec. 31, 2018	Oct. 31, 2018	Oct. 19, 2018
Class of Warrant or Right [Line Items]
Period between issuance and expiration of outstanding warrant									10 years
Fair value adjustment of warrants				$ 9,312	$ 4,059
Share price		$ 16.36	$ 27.30	$ 16.36	$ 27.30
Redeemable Preferred Stock
Class of Warrant or Right [Line Items]
Warrants to purchase common stock											534,240
Warrant exercise price per share											$ 0.01
Period between issuance and expiration of outstanding warrant											10 years
Fair value of warrants						$ 16,900				$ 12,800
Redeemable Preferred Stock \| Other Expense
Class of Warrant or Right [Line Items]
Fair value adjustment of warrants						4,100
Redeemable Series A-1 Preferred Stock
Class of Warrant or Right [Line Items]
Warrants to purchase common stock							534,240				534,240	534,240
Warrant exercise price per share							$ 0.01				$ 0.01
Period between issuance and expiration of outstanding warrant											10 years
Fair value of warrants						$ 16,900	$ 11,900
Redeemable Series A-1 Preferred Stock \| Other Expense
Class of Warrant or Right [Line Items]
Fair value adjustment of warrants		$ 5,000	$ (2,500)	$ 5,000	$ (4,100)
Redeemable Series A-2 Preferred Stock
Class of Warrant or Right [Line Items]
Warrants to purchase common stock							1,999,999	1,351,960
Warrant exercise price per share							$ 0.01	$ 0.01
Period between issuance and expiration of outstanding warrant								10 years
Fair value of warrants							$ 44,400
Aggregate value to determine number of shares underlying the warrant and issuable upon exercise of warrants		30,000		30,000
Redeemable Series A-2 Preferred Stock \| Other Expense
Class of Warrant or Right [Line Items]
Fair value adjustment of warrants		$ 14,300		$ 14,300
Convertible And Redeemable Series A-2 Preferred Stock
Class of Warrant or Right [Line Items]
Warrants to purchase common stock	1,999,999
Share price	$ 15.00
Increase of shares	648,039