Annual report pursuant to Section 13 and 15(d)

Significant Accounting Policies - Concentration of Risks (Details)

v3.21.2
Significant Accounting Policies - Concentration of Risks (Details) - Revenues - Customer Concentration Risk
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Public      
Concentration Risk [Line Items]      
Concentration risk percentage 61.30% 65.30% 69.30%
Private      
Concentration Risk [Line Items]      
Concentration risk percentage 38.70% 34.70% 30.70%
Department of Transportation      
Concentration Risk [Line Items]      
Concentration risk percentage 33.70% 32.50% 40.40%
Alabama Department of Transportation      
Concentration Risk [Line Items]      
Concentration risk percentage 10.80% 11.60% 13.80%
North Carolina Department of Transportation      
Concentration Risk [Line Items]      
Concentration risk percentage 10.30% 7.80% 13.10%