Business Segments (Total Assets For Reportable Segments) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Assets	$ 4,892,100	$ 5,045,617
Corporate and other
Segment Reporting Information [Line Items]
Assets	(73,660)	82,086
Civil | Operating Segments
Segment Reporting Information [Line Items]
Assets	3,253,753	3,141,991
Building | Operating Segments
Segment Reporting Information [Line Items]
Assets	1,052,374	1,147,649
Specialty Contractors | Operating Segments
Segment Reporting Information [Line Items]
Assets	$ 659,633	$ 673,891