Condensed Financial Information of Registrant Statements of Cash Flows (Detail) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Cash flows provided by operating activities:
Net cash provided by operating activities		$ 175,668	$ 97,382	$ 57,460
Cash flows provided by investing activities:
Net cash provided by investing activities		(28,141)	(82,166)	(65,252)
Cash flows used in financing activities:
Distributions		(24,451)	(25,593)	(25,603)
Unit repurchase		(38,431)	(51,353)	(26,714)
Net cash used in financing activities		(95,515)	(24,848)	(30,135)
Cash and cash equivalents at beginning of period		4,899
Cash and cash equivalents at end of period		56,911	4,899
STAR GROUP, L.P.
Cash flows provided by operating activities:
Net cash provided by operating activities	[1]	62,877	76,942	52,317
Cash flows provided by investing activities:
Net cash provided by investing activities		0	0	0
Cash flows used in financing activities:
Distributions		(24,451)	(25,593)	(25,603)
Unit repurchase		(38,431)	(51,353)	(26,714)
Net cash used in financing activities		(62,882)	(76,946)	(52,317)
Net decrease in cash		(5)	(4)
Cash and cash equivalents at beginning of period		50	54	54
Cash and cash equivalents at end of period		$ 45	$ 50	$ 54

[1] Includes distributions from subsidiaries