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Document and Entity Information - USD ($)
 12 Months Ended
Sep. 30, 2025
Nov. 30, 2025
Mar. 31, 2025
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Sep. 30, 2025    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
Trading Symbol SGU    
Title of 12(b) Security Common Units    
Security Exchange Name NYSE    
Entity Registrant Name STAR GROUP, L.P.    
Entity Central Index Key 0001002590    
Entity Tax Identification Number 06-1437793    
Current Fiscal Year End Date --09-30    
Entity Well-known Seasoned Issuer No    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Interactive Data Current Yes    
Entity Incorporation, State or Country Code DE    
Document Annual Report true    
Document Transition Report false    
Entity File Number 001-14129    
Entity Filer Category Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding   32,976,515  
Entity Public Float     $ 402,396,722
Entity Address, Address Line One 9 West Broad Street    
Entity Address, Address Line Two Suite 310    
Entity Address, City or Town Stamford    
Entity Address, State or Province CT    
Entity Address, Postal Zip Code 06902    
City Area Code (203)    
Local Phone Number 328-7310    
Auditor Name KPMG LLP    
Auditor Firm ID 185    
Auditor Location New York, NY    
ICFR Auditor Attestation Flag true    
Document Financial Statement Error Correction [Flag] false    
Auditor Opinion [Text Block]

Opinions on the Consolidated Financial Statements and Internal Control Over Financial Reporting

We have audited the accompanying consolidated balance sheets of Star Group, L.P. and subsidiaries (the Company) as of September 30, 2025 and 2024, the related consolidated statements of operations, comprehensive income, partners’ capital, and cash flows for each of the years in the three-year period ended September 30, 2025, and the related notes and financial statement schedules I to II (collectively, the consolidated financial statements). We also have audited the Company’s internal control over financial reporting as of September 30, 2025, based on criteria established in Internal Control – Integrated Framework (2013) issued by the Committee of Sponsoring Organizations of the Treadway Commission.

In our opinion, the consolidated financial statements referred to above present fairly, in all material respects, the financial position of the Company as of September 30, 2025 and 2024, and the results of its operations and its cash flows for each of the years in the three-year period ended September 30, 2025, in conformity with U.S. generally accepted accounting principles. Also in our opinion, the Company maintained, in all material respects, effective internal control over financial reporting as of September 30, 2025 based on criteria established in Internal Control – Integrated Framework (2013) issued by the Committee of Sponsoring Organizations of the Treadway Commission.