v3.26.1
Cover Page - shares
 9 Months Ended
Jun. 30, 2026
Jul. 31, 2026
Entity Information [Line Items]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2026  
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 false  
Entity Shell Company false  
Entity Central Index Key 0001644378  
Amendment Flag false  
Current Fiscal Year End Date --09-30  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q3  
Class A Common Stock    
Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding   16,080,226
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