Business Segments (Schedule of Assets by Segment) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Reportable segment information
Total Assets	$ 4,282,785	$ 4,038,620
Civil [Member]
Reportable segment information
Total Assets	2,273,486	2,152,123
Building [Member]
Reportable segment information
Total Assets	889,136	917,317
Specialty Contractors [Member]
Reportable segment information
Total Assets	757,103	813,851
Corporate and Other [Member]
Reportable segment information
Total Assets	$ 363,060	$ 155,329