Quarterly report pursuant to Section 13 or 15(d)

Significant Accounting Policies - Concentration of Risks (Detail)

v3.20.1
Significant Accounting Policies - Concentration of Risks (Detail) - Revenues - Customer Concentration Risk
 3 Months Ended 6 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Mar. 31, 2020
Mar. 31, 2019
Department of Transportation        
Concentration Risk [Line Items]        
Concentration risk percentage 27.80% 37.20% 29.40% 37.20%
Alabama Department of Transportation        
Concentration Risk [Line Items]        
Concentration risk percentage 8.80% 12.30% 9.80% 11.10%
North Carolina Department of Transportation        
Concentration Risk [Line Items]        
Concentration risk percentage 7.60% 10.70% 8.10% 12.60%