Annual report pursuant to Section 13 and 15(d)

Significant Accounting Policies - Concentration of Risks (Details)

v3.23.3
Significant Accounting Policies - Concentration of Risks (Details) - Revenues - Customer Concentration Risk
12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Public      
Concentration Risk [Line Items]      
Concentration risk percentage 63.00% 60.90% 61.30%
Private      
Concentration Risk [Line Items]      
Concentration risk percentage 37.00% 39.10% 38.70%
Departments of Transportation      
Concentration Risk [Line Items]      
Concentration risk percentage 36.20% 36.80% 33.70%
Alabama Department of Transportation      
Concentration Risk [Line Items]      
Concentration risk percentage   10.00% 10.80%
Florida Department of Transportation      
Concentration Risk [Line Items]      
Concentration risk percentage 10.70%    
North Carolina Department of Transportation      
Concentration Risk [Line Items]      
Concentration risk percentage 10.50% 11.20% 10.30%