Cover Page - shares	9 Months Ended
	Jun. 30, 2020	Aug. 06, 2020
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2020
Document Transition Report	false
Entity File Number	001-37616
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
Title of 12(b) Security	Class A common stock, $0.001 par value per share
Trading Symbol	RMR
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Central Index Key	0001644378
Amendment Flag	false
Current Fiscal Year End Date	--09-30
Current Fiscal Year End Date	2020
Document Fiscal Period Focus	Q3
Class A Common Stock
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		15,315,025
Class B-1 Common Stock
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		1,000,000
Class B-2 Common Stock
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		15,000,000