Quarterly report pursuant to Section 13 or 15(d)

Significant Accounting Policies - Consolidation of Revenues (Detail)

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Significant Accounting Policies - Consolidation of Revenues (Detail) - Revenues [Member] - Customer Concentration Risk [Member]
3 Months Ended 9 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Alabama Department of Transportation [Member]        
Concentration Risk [Line Items]        
Concentration risk percentage 17.00% 13.60% 14.80% 12.80%
North Carolina Department of Transportation [Member]        
Concentration Risk [Line Items]        
Concentration risk percentage 14.80% 16.60% 13.50% 13.20%