Annual report pursuant to Section 13 and 15(d)

Significant Accounting Policies - Concentration of Risks (Detail)

v3.20.2
Significant Accounting Policies - Concentration of Risks (Detail) - Revenues - Customer Concentration Risk
12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Public    
Concentration Risk [Line Items]    
Concentration risk percentage 65.30% 69.30%
Private    
Concentration Risk [Line Items]    
Concentration risk percentage 34.70% 30.70%
Department of Transportation    
Concentration Risk [Line Items]    
Concentration risk percentage 32.50% 40.40%
Alabama Department of Transportation    
Concentration Risk [Line Items]    
Concentration risk percentage 11.60% 13.80%
North Carolina Department of Transportation    
Concentration Risk [Line Items]    
Concentration risk percentage 7.80% 13.10%