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Summary of Significant Accounting Policies - Current and Long-Term Accounts Receivable and Notes Receivable and Allowance for Doubtful Accounts (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Accounting Policies [Abstract]    
Allowances for doubtful accounts on current receivable $ 6,662 $ 4,465
Current retainage balances 297,800 300,500
Non-current retainage balances 96,100 41,900
Unbilled receivables $ 460,300 $ 303,900