Business Segments (Schedule of Assets by Segment) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Reportable segment information
Total Assets	$ 4,357,765	$ 4,264,123
Civil [Member]
Reportable segment information
Total Assets	2,507,912	2,452,108
Building [Member]
Reportable segment information
Total Assets	913,031	909,207
Specialty Contractors [Member]
Reportable segment information
Total Assets	751,174	767,807
Corporate and Other [Member]
Reportable segment information
Total Assets	$ 185,648	$ 135,001