Business Segments (Reportable Segments) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Business Segments [Line items]
Revenue	$ 1,189,345	$ 1,123,137	$ 3,273,107	$ 3,271,378
Income (loss) from construction operations	47,943	47,306	(270,861)	101,196
Capital expenditures	23,331	16,108	62,677	64,411
Depreciation and amortization	16,227	12,417	44,541	33,581
External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	1,262,578	1,173,042	3,471,945	3,444,082
Elimination Of Intersegment Revenues [Member]
Business Segments [Line items]
Revenue	(73,233)	(49,905)	(198,838)	(172,704)
Reportable Segments [Member]
Business Segments [Line items]
Revenue	1,189,345	1,123,137	3,273,107	3,271,378
Income (loss) from construction operations	65,522	61,696	(225,165)	147,624
Capital expenditures	22,966	15,711	61,855	63,763
Depreciation and amortization	13,466	9,600	36,246	25,113
Reportable Segments [Member] | External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	1,262,578	1,173,042	3,471,945	3,444,082
Reportable Segments [Member] | Elimination Of Intersegment Revenues [Member]
Business Segments [Line items]
Revenue	(73,233)	(49,905)	(198,838)	(172,704)
Civil [Member]
Business Segments [Line items]
Revenue	524,546	431,488	1,331,698	1,097,075
Income (loss) from construction operations	50,695	41,282	(72,032)	93,560
Capital expenditures	22,497	15,364	60,948	61,912
Depreciation and amortization	11,953	8,031	31,608	20,356
Civil [Member] | External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	591,884	479,581	1,516,623	1,266,595
Civil [Member] | Elimination Of Intersegment Revenues [Member]
Business Segments [Line items]
Revenue	(67,338)	(48,093)	(184,925)	(169,520)
Building [Member]
Business Segments [Line items]
Revenue	415,346	455,492	1,277,130	1,392,712
Income (loss) from construction operations	7,580	8,853	6,903	27,814
Capital expenditures	144	277	349	1,147
Depreciation and amortization	495	488	1,495	1,458
Building [Member] | External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	421,241	457,304	1,291,043	1,395,896
Building [Member] | Elimination Of Intersegment Revenues [Member]
Business Segments [Line items]
Revenue	(5,895)	(1,812)	(13,913)	(3,184)
Specialty Contractors [Member]
Business Segments [Line items]
Revenue	249,453	236,157	664,279	781,591
Income (loss) from construction operations	7,247	11,561	(160,036)	26,250
Capital expenditures	325	70	558	704
Depreciation and amortization	1,018	1,081	3,143	3,299
Specialty Contractors [Member] | External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	249,453	236,157	664,279	781,591
Corporate [Member]
Business Segments [Line items]
Income (loss) from construction operations	(17,579)	(14,390)	(45,696)	(46,428)
Capital expenditures	365	397	822	648
Depreciation and amortization	$ 2,761	$ 2,817	$ 8,295	$ 8,468