Condensed Financial Information of Registrant of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2025	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Condensed Income Statements, Captions [Line Items]
Revenues	$ 247,692	$ 305,618	$ 743,045	$ 488,063	$ 240,331	$ 331,640	$ 666,032	$ 528,096	$ 1,784,418	$ 1,766,099	$ 1,952,862
General and administrative expenses									30,518	28,405	25,780
Operating income	(40,943)	(19,177)	125,372	49,219	(48,559)	(12,291)	100,331	21,621	114,471	61,102	62,545
Income before income taxes	(40,594)	(23,090)	120,678	45,908	(50,642)	(15,201)	96,244	18,153	102,902	48,554	45,929
Net income	$ (28,671)	$ (16,629)	$ 85,911	$ 32,884	$ (35,086)	$ (11,044)	$ 68,374	$ 12,979	73,495	35,223	31,945
STAR GROUP, L.P.
Condensed Income Statements, Captions [Line Items]
Revenues									0	0	0
General and administrative expenses									1,591	1,621	1,607
Operating income									(1,591)	(1,621)	(1,607)
Income before income taxes									(1,591)	(1,621)	(1,607)
Equity income of Star Acquisitions Inc. and subs									75,086	36,844	33,552
Net income									$ 73,495	$ 35,223	$ 31,945