Quarterly report pursuant to Section 13 or 15(d)

Significant Accounting Policies - Additional Information (Detail)

v3.10.0.1
Significant Accounting Policies - Additional Information (Detail)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2017
Jun. 30, 2018
Customer
Jun. 30, 2017
Customer
Sep. 30, 2017
USD ($)
Customer
Summary Of Accounting Policies [Line Items]          
Percentage of progress billings or contract price, to be withheld by customer until project completed in accordance with contract terms 10.00%   10.00%    
Prepaid Expense [Member]          
Summary Of Accounting Policies [Line Items]          
Capitalized equity issuance costs | $         $ 2.2
Additional Paid-in Capital [Member]          
Summary Of Accounting Policies [Line Items]          
Reclassified from prepaid expenses to additional paid-in capital | $ $ 6.3        
Revenues [Member] | Customer Concentration Risk [Member]          
Summary Of Accounting Policies [Line Items]          
Number of customers accounted for more than 10% | Customer     2 2  
Revenues [Member] | Customer Concentration Risk [Member] | Minimum [Member]          
Summary Of Accounting Policies [Line Items]          
Concentration risk percentage 10.00% 10.00% 10.00% 10.00%  
Revenues [Member] | Customer Concentration Risk [Member] | Department of Transportation [Member]          
Summary Of Accounting Policies [Line Items]          
Concentration risk percentage 45.60% 43.70% 40.90% 38.70%  
Contract Receivable Retainage [Member] | Credit Concentration Risk [Member]          
Summary Of Accounting Policies [Line Items]          
Number of customers accounted for more than 10% | Customer     0   0
Contract Receivable Retainage [Member] | Credit Concentration Risk [Member] | Maximum [Member]          
Summary Of Accounting Policies [Line Items]          
Concentration risk percentage     10.00%   10.00%