Business Segments (Schedule of Assets by Segment) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Reportable segment information
Total Assets	$ 4,164,866	$ 4,038,620
Civil [Member]
Reportable segment information
Total Assets	2,311,858	2,152,123
Building [Member]
Reportable segment information
Total Assets	878,501	917,317
Specialty Contractors [Member]
Reportable segment information
Total Assets	844,919	813,851
Corporate and Other [Member]
Reportable segment information
Total Assets	$ 129,588	$ 155,329