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Concentration of Risk and Enterprise Wide Disclosures (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Concentration Risk [Line Items]          
Contract revenues $ 125,073 $ 140,162 $ 421,682 $ 416,339  
Customer concentration risk | Trade and contract retainage receivables          
Concentration Risk [Line Items]          
Trade and retainage receivables $ 111,397   $ 111,397   $ 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 $ 125,073 $ 140,162 $ 421,682 $ 416,339  
Customer concentration risk | Federal Government | Trade and contract retainage receivables          
Concentration Risk [Line Items]          
Trade and retainage receivables $ 6,168   $ 6,168   $ 3,509
Concentration risk, percentage     5.00%   3.00%
Customer concentration risk | Federal Government | Contract revenues          
Concentration Risk [Line Items]          
Concentration risk, percentage 15.00% 10.00% 11.00% 13.00%  
Contract revenues $ 18,072 $ 14,464 $ 47,170 $ 52,556  
Customer concentration risk | State Governments | Trade and contract retainage receivables          
Concentration Risk [Line Items]          
Trade and retainage receivables $ 2,740   $ 2,740   $ 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% 7.00% 6.00% 8.00%  
Contract revenues $ 7,674 $ 9,185 $ 25,947 $ 33,910  
Customer concentration risk | Local Governments | Trade and contract retainage receivables          
Concentration Risk [Line Items]          
Trade and retainage receivables $ 13,781   $ 13,781   $ 18,256
Concentration risk, percentage     12.00%   15.00%
Customer concentration risk | Local Governments | Contract revenues          
Concentration Risk [Line Items]          
Concentration risk, percentage 20.00% 20.00% 17.00% 17.00%  
Contract revenues $ 25,546 $ 28,246 $ 69,298 $ 72,802  
Customer concentration risk | Private Companies | Trade and contract retainage receivables          
Concentration Risk [Line Items]          
Trade and retainage receivables $ 88,708   $ 88,708   $ 97,874
Concentration risk, percentage     81.00%   79.00%
Customer concentration risk | Private Companies | Contract revenues          
Concentration Risk [Line Items]          
Concentration risk, percentage 59.00% 63.00% 66.00% 62.00%  
Contract revenues $ 73,781 $ 88,267 $ 279,267 $ 257,071