Annual report pursuant to Section 13 and 15(d)

Significant Accounting Policies - Concentration of Risks (Detail)

v3.10.0.1
Significant Accounting Policies - Concentration of Risks (Detail) - Revenues - Customer Concentration Risk
12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Department of Transportation    
Concentration Risk [Line Items]    
Concentration risk percentage 42.90% 41.90%
Alabama Department of Transportation    
Concentration Risk [Line Items]    
Concentration risk percentage 15.10% 14.90%
North Carolina Department of Transportation    
Concentration Risk [Line Items]    
Concentration risk percentage 13.30% 13.90%