Quarterly report pursuant to Section 13 or 15(d)

Significant Accounting Policies (Details)

v3.19.1
Significant Accounting Policies (Details)
$ in Thousands
3 Months Ended 6 Months Ended
Apr. 23, 2018
Mar. 31, 2019
USD ($)
Mar. 31, 2018
USD ($)
Mar. 31, 2019
USD ($)
Mar. 31, 2018
USD ($)
Concentration Risk [Line Items]          
Revenues   $ 164,304 $ 118,899 $ 318,631 $ 269,320
Revenues | Customer Concentration Risk | Department of Transportation          
Concentration Risk [Line Items]          
Concentration risk percentage   37.20% 37.60% 37.20% 37.40%
Revenues | Customer Concentration Risk | Alabama Department of Transportation          
Concentration Risk [Line Items]          
Concentration risk percentage   12.30% 13.20% 11.10% 13.20%
Revenues | Customer Concentration Risk | North Carolina Department of Transportation          
Concentration Risk [Line Items]          
Concentration risk percentage   10.70% 11.80% 12.60% 12.60%
Private | Revenues | Customer Concentration Risk          
Concentration Risk [Line Items]          
Revenues   $ 50,419 $ 42,620 $ 99,986 $ 92,232
Public | Revenues | Customer Concentration Risk          
Concentration Risk [Line Items]          
Revenues   $ 113,885 $ 76,279 $ 218,645 $ 177,088
Class B Common Stock          
Concentration Risk [Line Items]          
Stock split, conversion ratio 25.2