Condensed Financial Information of Registrant Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Cash flows provided by operating activities:
Net cash provided by operating activities	$ 70,950	$ 110,978	$ 123,658
Cash flows provided by investing activities:
Net cash provided by investing activities	(99,854)	(61,185)	(28,197)
Cash flows used in financing activities:
Distributions	(26,116)	(25,038)	(23,744)
Unit repurchase	(15,595)	(11,088)	(4,475)
Net cash (used in) provided by financing activities	(63,748)	22,351	(64,890)
Net increase (decrease) in cash	(92,652)	72,144	30,571
Cash and cash equivalents at beginning of period	117,335
Cash and cash equivalents at end of period	24,683	117,335
STAR GROUP, L.P.
Cash flows provided by operating activities:
Net cash provided by operating activities	41,712	36,125	28,219
Cash flows provided by investing activities:
Net cash provided by investing activities	0	0	0
Cash flows used in financing activities:
Distributions	(26,116)	(25,038)	(23,744)
Unit repurchase	(15,595)	(11,088)	(4,475)
Net cash (used in) provided by financing activities	(41,711)	(36,126)	(28,219)
Net increase (decrease) in cash	1	(1)	0
Cash and cash equivalents at beginning of period	40	41	41
Cash and cash equivalents at end of period	$ 41	$ 40	$ 41