Business Segments (Reconciliation of Segment Results to Consolidated Income Before Income Taxes) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets	$ 5,045,617	$ 4,485,777
Corporate, Non-Segment
Assets	82,086	63,897
Civil | Operating Segments
Assets	3,141,991	2,791,402
Building | Operating Segments
Assets	1,147,649	995,298
Specialty Contractors | Operating Segments
Assets	$ 673,891	$ 635,180