Condensed Financial Information of Registrant of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Condensed Income Statements, Captions [Line Items]
Revenues	$ 266,937	$ 300,121	$ 737,617	$ 648,187	$ 296,644	$ 439,101	$ 782,543	$ 488,270	$ 1,952,862	$ 2,006,558	$ 1,497,086
General and administrative expenses									25,780	24,882	25,096
Operating income	(22,989)	(29,586)	91,515	23,605	(67,920)	(11,496)	117,245	22,624	62,545	60,453	130,200
Income before income taxes	(26,171)	(33,196)	86,294	19,002	(71,227)	(14,353)	114,279	20,327	45,929	49,026	121,412
Net income	$ (19,729)	$ (23,906)	$ 62,041	$ 13,539	$ (49,993)	$ (10,587)	$ 81,379	$ 14,489	31,945	35,288	87,737
STAR GROUP, L.P.
Condensed Income Statements, Captions [Line Items]
Revenues									0	0	0
General and administrative expenses									1,607	1,588	1,602
Operating income									(1,607)	(1,588)	(1,602)
Income before income taxes									(1,607)	(1,588)	(1,602)
Equity income of Star Acquisitions Inc. and subs									33,552	36,876	89,339
Net income									$ 31,945	$ 35,288	$ 87,737