Condensed Financial Information of Registrant Balance Sheets (Detail) - USD ($) $ in Thousands		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Current assets
Cash and cash equivalents		$ 4,767	$ 56,911
Prepaid expenses and other current assets		30,140	29,554
Total current assets		221,992	226,345
Total assets		853,863	838,616
Current liabilities
Accrued expenses		121,221	127,286
Total current liabilities		344,997	344,855
Partners’ capital		278,204	255,820	$ 260,840	$ 309,785
Total liabilities and partners’ capital		853,863	838,616
STAR GROUP, L.P.
Current assets
Cash and cash equivalents		45	45	$ 50	$ 54
Prepaid expenses and other current assets		353	313
Total current assets		398	358
Investment in subsidiaries	[1]	277,817	255,465
Total assets		278,215	255,823
Current liabilities
Accrued expenses		11	3
Total current liabilities		11	3
Partners’ capital		278,204	255,820
Total liabilities and partners’ capital		$ 278,215	$ 255,823

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