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Concentration of Risk and Enterprise Wide Disclosures (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Concentration Risk [Line Items]          
Contract revenues $ 159,767 $ 137,420 $ 296,610 $ 276,177  
Customer concentration risk | Trade and contract retainage receivables          
Concentration Risk [Line Items]          
Trade and retainage receivables $ 114,924   $ 114,924   $ 124,142
Concentration risk, percentage     100.00%   100.00%
Customer concentration risk | Contract revenues          
Concentration Risk [Line Items]          
Concentration risk, percentage 100.00% 100.00% 100.00% 100.00%  
Contract revenues $ 159,767 $ 137,420 $ 296,610 $ 276,177  
Customer concentration risk | Federal Government | Trade and contract retainage receivables          
Concentration Risk [Line Items]          
Trade and retainage receivables $ 2,907   $ 2,907   $ 3,509
Concentration risk, percentage     3.00%   3.00%
Customer concentration risk | Federal Government | Contract revenues          
Concentration Risk [Line Items]          
Concentration risk, percentage 10.00% 13.00% 10.00% 14.00%  
Contract revenues $ 16,077 $ 18,360 $ 29,100 $ 38,092  
Customer concentration risk | State Governments | Trade and contract retainage receivables          
Concentration Risk [Line Items]          
Trade and retainage receivables $ 2,713   $ 2,713   $ 4,503
Concentration risk, percentage     2.00%   3.00%
Customer concentration risk | State Governments | Contract revenues          
Concentration Risk [Line Items]          
Concentration risk, percentage 6.00% 9.00% 6.00% 9.00%  
Contract revenues $ 9,898 $ 12,706 $ 18,274 $ 24,726  
Customer concentration risk | Local Governments | Trade and contract retainage receivables          
Concentration Risk [Line Items]          
Trade and retainage receivables $ 19,910   $ 19,910   $ 18,256
Concentration risk, percentage     17.00%   15.00%
Customer concentration risk | Local Governments | Contract revenues          
Concentration Risk [Line Items]          
Concentration risk, percentage 13.00% 15.00% 15.00% 16.00%  
Contract revenues $ 20,522 $ 20,731 $ 43,752 $ 44,556  
Customer concentration risk | Private Companies | Trade and contract retainage receivables          
Concentration Risk [Line Items]          
Trade and retainage receivables $ 89,394   $ 89,394   $ 97,874
Concentration risk, percentage     78.00%   79.00%
Customer concentration risk | Private Companies | Contract revenues          
Concentration Risk [Line Items]          
Concentration risk, percentage 71.00% 63.00% 69.00% 61.00%  
Contract revenues $ 113,270 $ 85,623 $ 205,484 $ 168,803