Summary of Significant Accounting Policies - Current and Long-Term Accounts and Notes Receivable and Allowance for Doubtful Accounts (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Allowances for doubtful accounts on current receivable	$ 4,465	$ 2,752
Current retainage balances	300,500	231,000
Non-current retainage balances	41,900	5,200
Unbilled receivables	$ 303,900	$ 206,800