Business Segments (Total Assets For Reportable Segments) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Assets	$ 4,946,007	$ 5,045,617
Corporate and other
Segment Reporting Information [Line Items]
Assets	28,295	82,086
Civil | Operating Segments
Segment Reporting Information [Line Items]
Assets	3,225,802	3,141,991
Building | Operating Segments
Segment Reporting Information [Line Items]
Assets	1,032,928	1,147,649
Specialty Contractors | Operating Segments
Segment Reporting Information [Line Items]
Assets	$ 658,982	$ 673,891