Business Segments (Reportable Segments) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Business Segments [Line items]
Revenue	$ 1,120,085	$ 1,247,274	$ 2,148,241	$ 2,364,635
INCOME FROM CONSTRUCTION OPERATIONS	54,815	34,045	53,890	71,062
Capital expenditures	28,333	2,511	48,303	8,183
Depreciation and amortization	10,977	9,762	21,164	30,758
External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	1,180,222	1,329,178	2,271,040	2,522,852
Elimination Of Intersegment Revenues [Member]
Business Segments [Line items]
Revenue	(60,137)	(81,904)	(122,799)	(158,217)
Reportable Segments [Member]
Business Segments [Line items]
Revenue	1,120,085	1,247,274	2,148,241	2,364,635
INCOME FROM CONSTRUCTION OPERATIONS	69,429	49,873	85,928	101,764
Capital expenditures	28,159	2,240	48,052	7,858
Depreciation and amortization	8,164	6,942	15,513	24,970
Reportable Segments [Member] | External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	1,180,222	1,329,178	2,271,040	2,522,852
Reportable Segments [Member] | Elimination Of Intersegment Revenues [Member]
Business Segments [Line items]
Revenue	(60,137)	(81,904)	(122,799)	(158,217)
Civil [Member]
Business Segments [Line items]
Revenue	402,473	472,582	665,587	777,157
INCOME FROM CONSTRUCTION OPERATIONS	49,439	58,144	52,278	90,032
Capital expenditures	27,352	1,850	46,548	7,417
Depreciation and amortization	6,569	5,236	12,325	21,554
Civil [Member] | External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	461,614	538,552	787,014	905,363
Civil [Member] | Elimination Of Intersegment Revenues [Member]
Business Segments [Line items]
Revenue	(59,141)	(65,970)	(121,427)	(128,206)
Building [Member]
Business Segments [Line items]
Revenue	446,979	492,835	937,220	989,925
INCOME FROM CONSTRUCTION OPERATIONS	12,536	5,736	18,961	10,977
Capital expenditures	592	104	870	148
Depreciation and amortization	489	513	970	1,031
Building [Member] | External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	447,975	508,769	938,592	1,019,936
Building [Member] | Elimination Of Intersegment Revenues [Member]
Business Segments [Line items]
Revenue	(996)	(15,934)	(1,372)	(30,011)
Specialty Contractors [Member]
Business Segments [Line items]
Revenue	270,633	281,857	545,434	597,553
INCOME FROM CONSTRUCTION OPERATIONS	7,454	(14,007)	14,689	755
Capital expenditures	215	286	634	293
Depreciation and amortization	1,106	1,193	2,218	2,385
Specialty Contractors [Member] | External And Intersegment Customers [Member]
Business Segments [Line items]
Revenue	270,633	281,857	545,434	597,553
Corporate [Member]
Business Segments [Line items]
INCOME FROM CONSTRUCTION OPERATIONS	(14,614)	(15,828)	(32,038)	(30,702)
Capital expenditures	174	271	251	325
Depreciation and amortization	$ 2,813	$ 2,820	$ 5,651	$ 5,788