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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
Jun. 30, 2018
Dec. 31, 2017
Accounting Policies [Abstract]    
Allowances for doubtful accounts on current receivable $ 4,757 $ 4,465
Current retainage balances 254,800 300,500
Non-current retainage balances 90,700 41,900
Unbilled receivables $ 433,500 $ 303,900