Condensed Financial Information of Registrant Balance Sheets (Detail) - USD ($) $ in Thousands		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Current assets
Cash and cash equivalents		$ 56,911	$ 4,899
Prepaid expenses and other current assets		29,554	36,898
Total current assets		226,345	226,830
Total assets		838,616	752,706
Current liabilities
Accrued expenses		127,286	120,839
Total current liabilities		344,855	325,557
Partners’ capital		255,820	260,840	$ 309,785	$ 306,068
Total liabilities and partners’ capital		838,616	752,706
STAR GROUP, L.P.
Current assets
Cash and cash equivalents		45	50	$ 54	$ 54
Prepaid expenses and other current assets		313	232
Total current assets		358	282
Investment in subsidiaries	[1]	255,465	260,601
Total assets		255,823	260,883
Current liabilities
Accrued expenses		3	43
Total current liabilities		3	43
Partners’ capital		255,820	260,840
Total liabilities and partners’ capital		$ 255,823	$ 260,883

[1] Investments in Star Acquisitions, Inc. and subsidiaries are recorded in accordance with the equity method of accounting.