| Cover Page | Nov. 20, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Entity File Number | 001-37616 |
| Document Period End Date | Nov. 20, 2020 |
| Entity Registrant Name | RMR Group Inc. |
| Entity Incorporation, State or Country Code | MD |
| Entity Tax Identification Number | 47-4122583 |
| Entity Address, Address Line One | Two Newton Place |
| Entity Address, Address Line Two | 255 Washington Street |
| Entity Address, Address Line Three | Suite 300 |
| Entity Address, City or Town | Newton |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02458-1634 |
| City Area Code | 617 |
| Local Phone Number | 796-8230 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Class A common stock, $0.001 par value per share |
| Trading Symbol | RMR |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
| Entity Central Index Key | 0001644378 |
| Amendment Flag | false |
