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Accounts Receivable
 12 Months Ended
Dec. 31, 2016
Receivables [Abstract]  
Accounts Receivable
6. Accounts Receivable

Accounts receivable consist of the following (in thousands):
 
 
December 31,
 
 
2016
 
2015
Contracts in progress
 
$
121,666

 
$
103,178

Unbilled retentions
 
35,069

 
31,195

Trade receivables
 
113,380

 
77,084

Other receivables
 
1,102

 
190

Allowance for doubtful accounts
 
(3,619
)
 
(794
)
Total accounts receivable
 
$
267,598

 
$
210,853