Business Segments (Schedule of Assets by Segment) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Reportable segment information
Total Assets	$ 4,220,966	$ 4,036,677
Civil [Member]
Reportable segment information
Total Assets	2,082,218	1,962,503
Building [Member]
Reportable segment information
Total Assets	895,198	797,386
Specialty Contractors [Member]
Reportable segment information
Total Assets	826,144	860,285
Corporate and Other [Member]
Reportable segment information
Total Assets	$ 417,406	$ 416,503