Annual report pursuant to Section 13 and 15(d)

Significant Accounting Policies - Concentration of Risks (Details)

v3.22.2.2
Significant Accounting Policies - Concentration of Risks (Details) - Revenues - Customer Concentration Risk
12 Months Ended
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2020
Public      
Concentration Risk [Line Items]      
Concentration risk percentage 60.90% 61.30% 65.30%
Private      
Concentration Risk [Line Items]      
Concentration risk percentage 39.10% 38.70% 34.70%
Departments of Transportation      
Concentration Risk [Line Items]      
Concentration risk percentage 36.80% 33.70% 32.50%
Alabama Department of Transportation      
Concentration Risk [Line Items]      
Concentration risk percentage 10.00% 10.80% 11.60%
North Carolina Department of Transportation      
Concentration Risk [Line Items]      
Concentration risk percentage 11.20% 10.30% 7.80%