Quarterly report pursuant to Section 13 or 15(d)

Significant Accounting Policies (Details)

v3.24.0.1
Significant Accounting Policies (Details) - USD ($)
3 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Sep. 30, 2023
Concentration Risk [Line Items]      
Restricted cash $ 973,000   $ 837,000
Restricted investments 14,585,000   $ 15,079,000
Debt securities, available-for-sale, allowance for credit loss $ 0 $ 0  
Revenues | Customer Concentration Risk | Public      
Concentration Risk [Line Items]      
Concentration risk percentage 59.80% 61.20%  
Revenues | Customer Concentration Risk | Private      
Concentration Risk [Line Items]      
Concentration risk percentage 40.20% 38.80%  
Revenues | Customer Concentration Risk | Department of Transportation      
Concentration Risk [Line Items]      
Concentration risk percentage 37.70% 33.70%  
Revenues | Customer Concentration Risk | Florida Department of Transportation      
Concentration Risk [Line Items]      
Concentration risk percentage 12.60%    
Revenues | Customer Concentration Risk | North Carolina Department of Transportation      
Concentration Risk [Line Items]      
Concentration risk percentage   10.90%