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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, 2018 | Sep. 30, 2017 | Sep. 30, 2016
STAR GROUP, L.P. |  |  |
Condensed Cash Flow Statements, Captions [Line Items] |  |  |
Includes distributions from subsidiaries | $ 52,317 | $ 24,052 | $ 35,109

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