Quarterly report pursuant to Section 13 or 15(d)

Significant Accounting Policies - Revenue by Major Customers (Details)

v3.21.2
Significant Accounting Policies - Revenue by Major Customers (Details) - Revenues - Customer Concentration Risk
3 Months Ended 9 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Department of Transportation        
Concentration Risk [Line Items]        
Concentration risk percentage 35.90% 36.80% 30.70% 32.30%
Alabama Department of Transportation        
Concentration Risk [Line Items]        
Concentration risk percentage 10.90% 13.20% 9.30% 11.10%
North Carolina Department of Transportation        
Concentration Risk [Line Items]        
Concentration risk percentage 12.20% 9.00% 8.70% 8.40%
Private        
Concentration Risk [Line Items]        
Concentration risk percentage 38.50% 30.90% 40.00% 36.10%
Public        
Concentration Risk [Line Items]        
Concentration risk percentage 61.50% 69.10% 60.00% 63.90%