Condensed Financial Information of Registrant Balance Sheets (Detail) - USD ($) $ in Thousands		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Current assets
Cash and cash equivalents		$ 14,531	$ 52,458
Prepaid expenses and other current assets		35,451	26,652
Total current assets		256,737	241,241
Total assets		729,971	673,917
Current liabilities
Accrued expenses		116,436	108,449
Total current liabilities		283,188	272,333
Partners’ capital		309,785	306,068	$ 301,493	$ 289,886
Total liabilities and partners’ capital		729,971	673,917
STAR GROUP, L.P.
Current assets
Cash and cash equivalents		54	54	$ 324	$ 324
Prepaid expenses and other current assets		217	207
Total current assets		271	261
Investment in subsidiaries	[1]	309,541	306,016
Total assets		309,812	306,277
Current liabilities
Accrued expenses		27	209
Total current liabilities		27	209
Partners’ capital		309,785	306,068
Total liabilities and partners’ capital		$ 309,812	$ 306,277

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