Business Segments (Schedule of Assets by Segment) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Reportable segment information
Total Assets	$ 3,998,483	$ 4,038,620
Civil [Member]
Reportable segment information
Total Assets	2,102,432	2,152,123
Building [Member]
Reportable segment information
Total Assets	896,442	917,317
Specialty Contractors [Member]
Reportable segment information
Total Assets	793,507	813,851
Corporate and Other [Member]
Reportable segment information
Total Assets	$ 206,102	$ 155,329