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Accounts Receivable (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Receivables [Abstract]          
Trade accounts receivable $ 124,609   $ 140,209    
Costs in excess of billings 22,634   11,610    
Total contract receivables 147,243   151,819    
Less allowance for doubtful accounts (6,960)   (6,434) $ (5,272) $ (4,868)
Accounts receivable $ 140,283 $ 150,307 $ 145,385