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Goodwill and Intangible Assets - Goodwill (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Sep. 30, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Goodwill        
Goodwill, Beginning Balance   $ 153,374 $ 127,226  
Goodwill acquired during the year   52,785 26,269  
Goodwill impairment $ (2,700) 0 0 $ (2,716)
Purchase price allocation adjustments     121  
Goodwill, Ending Balance   206,159 153,374 127,226
Power        
Goodwill        
Goodwill, Beginning Balance   24,391 24,512  
Goodwill acquired during the year   1,542    
Purchase price allocation adjustments     121  
Goodwill, Ending Balance   25,933 24,391 24,512
Pipeline        
Goodwill        
Goodwill, Beginning Balance   51,521 42,252  
Goodwill acquired during the year   764 9,269  
Goodwill, Ending Balance   52,285 51,521 42,252
Utilities        
Goodwill        
Goodwill, Beginning Balance   37,312 20,312  
Goodwill acquired during the year     17,000  
Goodwill, Ending Balance   37,312 37,312 20,312
Transmission        
Goodwill        
Goodwill acquired during the year   50,479    
Goodwill, Ending Balance   50,479    
Civil        
Goodwill        
Goodwill, Beginning Balance   40,150 40,150  
Goodwill, Ending Balance   $ 40,150 $ 40,150 $ 40,150