Business Segments (Total Assets for Reportable Segments) (Details) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Assets		$ 4,686,406	$ 4,485,777
Civil [Member]
Segment Reporting Information [Line Items]
Assets		2,877,417	2,791,402
Building [Member]
Segment Reporting Information [Line Items]
Assets		1,094,207	995,298
Specialty Contractors [Member]
Segment Reporting Information [Line Items]
Assets		659,629	635,180
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Assets	[1]	$ 55,153	$ 63,897

[1] Consists principally of cash, equipment, tax-related assets and insurance-related assets, offset by the elimination of assets related to intersegment revenue.