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Summary of Significant Accounting Policies - Current and Long-Term Accounts and Notes Receivable and Allowance for Doubtful Accounts (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Accounting Policies [Abstract]    
Allowances for doubtful accounts on current receivable $ 5,066 $ 2,752
Current retainage balances 298,400 231,000
Non-current retainage balances 26,800 5,200
Unbilled receivables $ 393,000 $ 206,800