Condensed Financial Information of Registrant of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								9 Months Ended	12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Condensed Income Statements, Captions [Line Items]
Revenues	$ 229,618	$ 327,354	$ 684,031	$ 436,834	$ 181,584	$ 225,801	$ 532,052	$ 384,118		$ 1,677,837	$ 1,323,555	$ 1,161,338
General and administrative expenses										24,227	24,998	23,366
Operating income	(41,654)	(8,817)	85,473	31,066	(27,126)	(19,811)	69,032	33,237		66,068	55,332	87,404
Income before income taxes	(36,925)	(11,421)	82,783	28,670	(29,092)	(21,766)	66,996	31,138	$ 34,400	63,107	47,276	78,672
Net income	$ (21,450)	$ (8,005)	$ 54,778	$ 30,182	$ (17,747)	$ (13,332)	$ 39,704	$ 18,275		55,505	26,900	44,934
STAR GROUP, L.P.
Condensed Income Statements, Captions [Line Items]
Revenues										0	0	0
General and administrative expenses										1,647	2,116	1,363
Operating income										(1,647)	(2,116)	(1,363)
Income before income taxes										(1,647)	(2,116)	(1,363)
Equity income of Star Acquisitions Inc. and subs										57,152	29,016	46,297
Net income										$ 55,505	$ 26,900	$ 44,934