Business Segments (Schedule of Assets by Segment) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Reportable segment information
Total Assets	$ 4,425,168	$ 4,264,123
Civil [Member]
Reportable segment information
Total Assets	2,597,653	2,452,108
Building [Member]
Reportable segment information
Total Assets	936,365	909,207
Specialty Contractors [Member]
Reportable segment information
Total Assets	760,017	767,807
Corporate and Other [Member]
Reportable segment information
Total Assets	$ 131,133	$ 135,001