Business Segments (Total Assets For Reportable Segments) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Assets	$ 5,042,256	$ 4,485,777
Corporate and other
Segment Reporting Information [Line Items]
Assets	78,440	63,897
Civil | Operating Segments
Segment Reporting Information [Line Items]
Assets	3,168,681	2,791,402
Building | Operating Segments
Segment Reporting Information [Line Items]
Assets	1,086,462	995,298
Specialty Contractors | Operating Segments
Segment Reporting Information [Line Items]
Assets	$ 708,673	$ 635,180