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Condensed Financial Information of Registrant Statements of Cash Flows (Parenthetical) (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
STAR GROUP, L.P. |  |  |
Condensed Cash Flow Statements, Captions [Line Items] |  |  |
Includes distributions from subsidiaries | $ 62,877 | $ 76,942 | $ 52,317

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