Business Segments (Reportable Segments) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Business Segments [Line items]
Revenue	$ 1,123,137	$ 1,199,505	$ 3,271,378	$ 3,564,140
INCOME FROM CONSTRUCTION OPERATIONS	47,306	49,072	101,196	120,134
Capital expenditures	16,108	1,529	64,411	9,712
Depreciation and amortization	12,417	9,705	33,581	40,463
External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	1,173,042	1,269,044	3,444,082	3,791,896
Elimination Of Intersegment Revenues [Member]
Business Segments [Line items]
Revenue	(49,905)	(69,539)	(172,704)	(227,756)
Reportable Segments [Member]
Business Segments [Line items]
Revenue	1,123,137	1,199,505	3,271,378	3,564,140
INCOME FROM CONSTRUCTION OPERATIONS	61,696	66,777	147,624	168,541
Capital expenditures	15,711	1,365	63,763	9,223
Depreciation and amortization	9,600	6,881	25,113	31,851
Reportable Segments [Member] | External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	1,173,042	1,269,044	3,444,082	3,791,896
Reportable Segments [Member] | Elimination Of Intersegment Revenues [Member]
Business Segments [Line items]
Revenue	(49,905)	(69,539)	(172,704)	(227,756)
Civil [Member]
Business Segments [Line items]
Revenue	431,488	395,820	1,097,075	1,172,977
INCOME FROM CONSTRUCTION OPERATIONS	41,282	38,144	93,560	128,176
Capital expenditures	15,364	1,248	61,912	8,665
Depreciation and amortization	8,031	5,213	20,356	26,767
Civil [Member] | External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	479,581	458,487	1,266,595	1,363,850
Civil [Member] | Elimination Of Intersegment Revenues [Member]
Business Segments [Line items]
Revenue	(48,093)	(62,667)	(169,520)	(190,873)
Building [Member]
Business Segments [Line items]
Revenue	455,492	493,548	1,392,712	1,483,473
INCOME FROM CONSTRUCTION OPERATIONS	8,853	14,058	27,814	25,035
Capital expenditures	277	36	1,147	184
Depreciation and amortization	488	502	1,458	1,533
Building [Member] | External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	457,304	500,420	1,395,896	1,520,356
Building [Member] | Elimination Of Intersegment Revenues [Member]
Business Segments [Line items]
Revenue	(1,812)	(6,872)	(3,184)	(36,883)
Specialty Contractors [Member]
Business Segments [Line items]
Revenue	236,157	310,137	781,591	907,690
INCOME FROM CONSTRUCTION OPERATIONS	11,561	14,575	26,250	15,330
Capital expenditures	70	81	704	374
Depreciation and amortization	1,081	1,166	3,299	3,551
Specialty Contractors [Member] | External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	236,157	310,137	781,591	907,690
Corporate [Member]
Business Segments [Line items]
INCOME FROM CONSTRUCTION OPERATIONS	(14,390)	(17,705)	(46,428)	(48,407)
Capital expenditures	397	164	648	489
Depreciation and amortization	$ 2,817	$ 2,824	$ 8,468	$ 8,612