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Customer Concentrations (Details)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2018
USD ($)
customer
item
Dec. 31, 2017
USD ($)
customer
Dec. 31, 2016
USD ($)
Customer concentrations                      
Amount $ 877,670 $ 908,902 $ 648,787 $ 504,119 $ 579,017 $ 608,311 $ 631,165 $ 561,502 $ 2,939,478 $ 2,379,995 $ 1,996,948
Unbilled revenue 249,577       $ 160,092       249,577 160,092  
Utilities                      
Customer concentrations                      
Amount                 902,772 806,523 637,212
Civil                      
Customer concentrations                      
Amount                 464,972 501,777 479,152
Pipeline                      
Customer concentrations                      
Amount                 590,937 465,570 401,931
Power                      
Customer concentrations                      
Amount                 694,048 606,125 478,653
Utility Company under Chapter 11                      
Customer concentrations                      
Unbilled revenue $ 36,000               36,000    
Revenues | Customer concentration                      
Customer concentrations                      
Amount                 $ 1,038,378 $ 912,725 $ 910,007
Percentage                 35.30% 38.40% 45.60%
Revenues | Customer concentration | Public gas and electric utility | Utilities/Power                      
Customer concentrations                      
Amount                 $ 250,286 $ 210,747 $ 184,002
Percentage                 8.50% 8.90% 9.20%
Revenues | Customer concentration | Private gas and electric utility | Utilities                      
Customer concentrations                      
Amount                 $ 232,162 $ 190,659 $ 201,443
Percentage                 7.90% 8.00% 10.10%
Revenues | Customer concentration | State DOT | Civil                      
Customer concentrations                      
Amount                 $ 202,452 $ 222,142 $ 193,049
Percentage                 6.90% 9.30% 9.70%
Revenues | Customer concentration | Pipeline Operator | Pipeline                      
Customer concentrations                      
Amount                 $ 198,198    
Percentage                 6.70%    
Revenues | Customer concentration | Pipeline Operator | Maximum | Pipeline                      
Customer concentrations                      
Percentage                   5.00% 5.00%
Revenues | Customer concentration | Pipeline Operator | Pipeline/Power                      
Customer concentrations                      
Amount                 $ 155,280    
Percentage                 5.30%   5.00%
Revenues | Customer concentration | Pipeline Operator | Maximum | Pipeline/Power                      
Customer concentrations                      
Percentage                   5.00%  
Revenues | Customer concentration | Chemical/Energy Producer | Power/Civil                      
Customer concentrations                      
Amount                   $ 160,995 $ 208,458
Percentage                   6.80% 10.40%
Revenues | Customer concentration | Chemical/Energy Producer | Maximum | Power/Civil                      
Customer concentrations                      
Percentage                 5.00%    
Revenues | Customer concentration | Pipeline Operator | Pipeline                      
Customer concentrations                      
Amount                   $ 128,182  
Revenues | Customer concentration | Pipeline Operator | Maximum | Pipeline                      
Customer concentrations                      
Percentage                 5.00% 5.40% 5.00%
Revenues | Customer concentration | Pipeline Operator | Pipeline                      
Customer concentrations                      
Amount                     $ 123,055
Percentage                     6.20%
Revenues | Customer concentration | Pipeline Operator | Maximum | Pipeline                      
Customer concentrations                      
Percentage                 5.00% 5.00%  
Revenues | Customer concentration | Utility Company under Chapter 11                      
Customer concentrations                      
Percentage                 8.50%    
Revenues | Customer concentration | Top ten customers                      
Customer concentrations                      
Percentage                 52.20% 56.40% 60.40%
Number of top customers | customer                 10    
Number of calendar years in which top customers typically generate minimum specified percentage of revenue | item                 1    
Minimum percentage of revenues generated by top ten customers                 50.00%    
Revenues | Customer concentration | One customer                      
Customer concentrations                      
Percentage                 5.30% 8.90%  
Accounts receivable | Customer concentration | Utility Company under Chapter 11                      
Customer concentrations                      
Percentage                 1.90%    
Accounts receivable | Customer concentration | One customer                      
Customer concentrations                      
Percentage                 9.30% 4.30%  
Number of customers | customer                 1 1