Business Segments (Schedule of Assets by Segment) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Reportable segment information
Total Assets	$ 4,494,908	$ 4,387,752
Civil [Member]
Reportable segment information
Total Assets	2,791,537	2,574,326
Building [Member]
Reportable segment information
Total Assets	965,917	913,746
Specialty Contractors [Member]
Reportable segment information
Total Assets	644,016	745,313
Corporate and Other [Member]
Reportable segment information
Total Assets	$ 93,438	$ 154,367