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RETIREMENT PLANS - NARRATIVE (Details) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Defined Benefit Plan, Equity Securities, Micro Cap    
Retirement plans    
Market capitalization, percentage of equity securities 61.00%  
Defined Benefit Plan, Equity Securities, Small Cap    
Retirement plans    
Market capitalization, percentage of equity securities 22.00%  
Defined Benefit Plan, Equity Securities, Mid Cap    
Retirement plans    
Market capitalization, percentage of equity securities 8.00%  
Defined Benefit Plan, Equity Securities, Large Cap    
Retirement plans    
Market capitalization, percentage of equity securities 9.00%  
Pension Plan    
Retirement plans    
Period of employee's highest average earnings on which benefits are based 5 years  
Accumulated benefit obligation $ 8,101 $ 8,195
Expected future contributions $ 0  
Number of Barnwell common stock held by the Pension Plan (in shares) 666,077 413,148
SERP    
Retirement plans    
Accumulated benefit obligation $ 2,061 $ 1,974
Expected future contributions $ 270