CONTINGENT EARN-OUT AWARDS - Rollforward of Contingent Earn-out Liability (Details) $ in Thousands	9 Months Ended
Sep. 30, 2022 USD ($)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning	$ 377,576
Reallocation of Earn-out Shares to earn-out liability upon forfeitures	896
Change in fair value of earn-out liability	(136,043)
Issuance of Earn-out Shares upon triggering events	(242,429)
Balance at ending	$ 0

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Fair Value Adjustment

No definition available.

Name:	mttr_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityFairValueAdjustment
Namespace Prefix:	mttr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuance of Earn-out Shares Upon Triggering Events

+ References

No definition available.

+ Details

Name:	mttr_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssuanceOfEarnOutSharesUponTriggeringEvents
Namespace Prefix:	mttr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Reallocation upon Forfeitures of Shares

+ References

No definition available.

+ Details

Name:	mttr_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityReallocationOfEarnOutSharesToEarnOutLiabilityUponForfeitures
Namespace Prefix:	mttr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-SubTopic 10
-Topic 820
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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