Business Segments (Total Assets For Reportable Segments) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Assets	$ 4,827,750	$ 5,045,617
Corporate and other
Segment Reporting Information [Line Items]
Assets	(137,450)	82,086
Civil | Operating Segments
Segment Reporting Information [Line Items]
Assets	3,255,994	3,141,991
Building | Operating Segments
Segment Reporting Information [Line Items]
Assets	1,054,260	1,147,649
Specialty Contractors | Operating Segments
Segment Reporting Information [Line Items]
Assets	$ 654,946	$ 673,891