Annual report pursuant to Section 13 and 15(d)

Significant Accounting Policies - Concentration of Risks (Detail)

v3.19.3.a.u2
Significant Accounting Policies - Concentration of Risks (Detail) - Revenues - Customer Concentration Risk
12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Department of Transportation    
Concentration Risk [Line Items]    
Concentration risk percentage 40.40% 42.90%
North Carolina Department of Transportation    
Concentration Risk [Line Items]    
Concentration risk percentage 13.10% 13.30%
Alabama Department of Transportation    
Concentration Risk [Line Items]    
Concentration risk percentage 13.80% 15.10%
Private    
Concentration Risk [Line Items]    
Concentration risk percentage 30.70% 28.60%
Public    
Concentration Risk [Line Items]    
Concentration risk percentage 69.30% 71.40%