Quarterly report pursuant to Section 13 or 15(d)

Significant Accounting Policies - Additional Information (Detail)

v3.8.0.1
Significant Accounting Policies - Additional Information (Detail)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2018
USD ($)
Mar. 31, 2017
Mar. 31, 2018
USD ($)
Customer
Mar. 31, 2017
Customer
Sep. 30, 2017
USD ($)
Customer
Summary Of Accounting Policies [Line Items]          
Common stock split conversion ratio     25.2    
Stockholders equity note, stock split     Common share and per share amounts have been retroactively adjusted for all periods presented to give effect to the 25.2 to 1 split of the common stock (the "Stock Split")    
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 | $ $ 4.0   $ 4.0   $ 2.2
Revenues [Member] | Customer Concentration Risk [Member]          
Summary Of Accounting Policies [Line Items]          
Number of customers accounted for more than 10%     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 37.60% 33.90% 37.40% 35.40%  
Contract Receivable Retainage [Member] | Credit Concentration Risk [Member]          
Summary Of Accounting Policies [Line Items]          
Number of customers accounted for more than 10%     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%