Revenue (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Contracts Revenue	$ 1,189,345	$ 1,123,137	$ 3,273,107	$ 3,271,378
Civil [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	524,546	431,488	1,331,698	1,097,075
Civil [Member] | Mass Transit [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	265,671	177,619	655,541	485,841
Civil [Member] | Bridges [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	113,743	128,240	269,517	311,979
Civil [Member] | Highways [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	44,630	46,553	145,917	129,619
Civil [Member] | Tunneling [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	32,076	33,377	96,602	66,009
Civil [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	68,426	45,699	164,121	103,627
Building [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	415,346	455,492	1,277,130	1,392,712
Building [Member] | Commercial And Industrial Facilities [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	121,206	57,505	345,752	290,571
Building [Member] | Health Care Facilities [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	58,323	127,219	199,347	320,416
Building [Member] | Municipal And Government [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	62,444	67,003	192,986	187,984
Building [Member] | Education Facilities [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	33,469	43,405	123,059	108,763
Building [Member] | Hospitality And Gaming [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	57,672	65,744	180,556	226,999
Building [Member] | Mass Transit [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	53,384		123,772
Building [Member] | Mixed Use [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	2,183	40,758	26,774	121,348
Building [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	26,665	53,858	84,884	136,631
Specialty Contractors [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	249,453	236,157	664,279	781,591
Specialty Contractors [Member] | Commercial And Industrial Facilities [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	51,471	48,601	139,112	136,892
Specialty Contractors [Member] | Health Care Facilities [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	5,217	12,873	26,116	43,861
Specialty Contractors [Member] | Education Facilities [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	21,610	26,024	47,226	77,626
Specialty Contractors [Member] | Mass Transit [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	103,710	63,457	285,121	216,808
Specialty Contractors [Member] | Mixed Use [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	18,465	37,587	47,170	137,420
Specialty Contractors [Member] | Transportation [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	5,931	17,150	16,092	73,154
Specialty Contractors [Member] | Multi-Unit Residential [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	25,860	18,254	56,474	64,104
Specialty Contractors [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Contracts Revenue	$ 17,189	$ 12,211	$ 46,968	$ 31,726