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 6 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2018
Mar. 31, 2017
Alabama Department of Transportation [Member]        
Concentration Risk [Line Items]        
Concentration risk percentage 13.20% 10.70% 13.20% 12.30%
North Carolina Department of Transportation [Member]        
Concentration Risk [Line Items]        
Concentration risk percentage 11.80% 11.10% 12.60% 11.00%