Condensed Financial Information of Registrant of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Condensed Income Statements, Captions [Line Items]
Revenues	$ 236,551	$ 283,100	$ 604,115	$ 373,320	$ 183,295	$ 232,155	$ 543,063	$ 508,945	$ 1,497,086	$ 1,467,458	$ 1,753,872
General and administrative expenses									25,096	25,072	28,414
Operating income	(30,517)	(13,764)	119,695	54,786	(36,098)	498	86,117	42,451	130,200	92,968	37,350
Income before income taxes	(32,629)	(15,963)	117,316	52,688	(44,051)	(2,051)	83,108	39,537	121,412	76,543	25,154
Net income	$ (23,233)	$ (12,054)	$ 85,164	$ 37,860	$ (30,199)	$ (46)	$ 58,408	$ 27,755	87,737	55,918	17,637
STAR GROUP, L.P.
Condensed Income Statements, Captions [Line Items]
Revenues									0	0	0
General and administrative expenses									1,602	1,327	1,377
Operating income									(1,602)	(1,327)	(1,377)
Income before income taxes									(1,602)	(1,327)	(1,377)
Equity income of Star Acquisitions Inc. and subs									89,339	57,245	19,014
Net income									$ 87,737	$ 55,918	$ 17,637