Related Person Transactions - Tax Related Payments (Details) - USD ($) $ in Thousands	6 Months Ended
Mar. 31, 2021	Mar. 31, 2020
RMR LLC
Related Party Transaction [Line Items]
Tax distributions	$ 8,147	$ 8,806
ABP Trust \| RMR LLC
Related Party Transaction [Line Items]
Tax distributions	$ 7,279	7,986
ABP Trust \| Up C Transaction
Related Party Transaction [Line Items]
Tax receivable agreement, percent of payment	85.00%
Liability related to tax receivable agreement	$ 29,950
Current portion of liability related to tax receivable agreement	2,161
ABP Trust and Managed REIT \| RMR LLC
Related Party Transaction [Line Items]
Tax distributions	$ 15,426	$ 16,792

Tax Receivable Agreement Obligations, Current

No definition available.

Name:	rmr_TaxReceivableAgreementObligationsCurrent
Namespace Prefix:	rmr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the carrying amount on the reporting date of the current and non current obligations relating to the tax receivable agreement.

+ References

No definition available.

+ Details

Name:	rmr_TaxReceivableAgreementObligationsCurrentAndNoncurrent
Namespace Prefix:	rmr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the percent of cash savings, if any, in US Federal, state and local income tax or franchise tax provided in the Tax Receivable Agreement with a related party.

+ References

No definition available.

+ Details

Name:	rmr_TaxReceivableAgreementPaymentPercent
Namespace Prefix:	rmr_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cash distribution paid to unit-holder of limited liability company (LLC).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

+ Details

Name:	us-gaap_DistributionMadeToLimitedLiabilityCompanyLLCMemberCashDistributionsPaid
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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

- Details

Name:	dei_LegalEntityAxis=rmr_RMRLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=rmr_AbpTrustMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=rmr_UpCTransactionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=rmr_AbpTrustAndManagedReitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: