Business Segments (Reportable Segments) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Segments
Revenues		$ 1,193,068	$ 1,199,505	$ 1,247,274	$ 1,117,361	$ 1,246,599	$ 1,332,978	$ 1,308,130	$ 1,085,369	$ 4,757,208	$ 4,973,076	$ 4,920,472
Income from construction operations		59,343	$ 49,072	$ 34,045	$ 37,017	52,050	$ 60,919	$ 48,829	$ 40,122	179,477	201,920	105,413
Capital Expenditures										30,280	15,743	35,912
Depreciation and amortization	[1]									51,930	67,302	41,634
Assets		4,264,123				4,038,620				4,264,123	4,038,620
Intersubsegment Eliminations [Member]
Business Segments
Revenues										(295,521)	(238,800)	(213,243)
External and Intersegment Customers [Member]
Business Segments
Revenues										5,052,729	5,211,876	5,133,715
Corporate
Business Segments
Assets	[2]	135,001				155,329				135,001	155,329
Reportable Segments [Member]
Business Segments
Revenues										4,757,208	4,973,076	4,920,472
Income from construction operations										245,344	262,140	159,655
Capital Expenditures										28,682	15,062	12,453
Depreciation and amortization	[1]									40,487	55,782	30,836
Reportable Segments [Member] | Intersubsegment Eliminations [Member]
Business Segments
Revenues										(295,521)	(238,800)	(213,243)
Reportable Segments [Member] | External and Intersegment Customers [Member]
Business Segments
Revenues										5,052,729	5,211,876	5,133,715
Civil
Business Segments
Revenues										1,602,175	1,668,963	1,889,907
Income from construction operations										192,207	172,668	145,213
Capital Expenditures										27,694	13,541	8,383
Depreciation and amortization	[1]									33,767	48,561	22,601
Assets		2,452,108				2,152,123				2,452,108	2,152,123
Civil | Intersubsegment Eliminations [Member]
Business Segments
Revenues										(253,989)	(161,894)	(115,286)
Civil | External and Intersegment Customers [Member]
Business Segments
Revenues										1,856,164	1,830,857	2,005,193
Building
Business Segments
Revenues										1,941,325	2,069,841	1,802,535
Income from construction operations										34,199	51,564	(1,240)
Capital Expenditures										267	516	2,877
Depreciation and amortization	[1]									2,021	2,186	2,728
Assets		909,207				917,317				909,207	917,317
Building | Intersubsegment Eliminations [Member]
Business Segments
Revenues										(41,532)	(76,906)	(97,957)
Building | External and Intersegment Customers [Member]
Business Segments
Revenues										1,982,857	2,146,747	1,900,492
Specialty Contractors
Business Segments
Revenues										1,213,708	1,234,272	1,228,030
Income from construction operations										18,938	37,908	15,682
Capital Expenditures										721	1,005	1,193
Depreciation and amortization	[1]									4,699	5,035	5,507
Assets		$ 767,807				$ 813,851				767,807	813,851
Specialty Contractors | External and Intersegment Customers [Member]
Business Segments
Revenues										1,213,708	1,234,272	1,228,030
Corporate [Member]
Business Segments
Income from construction operations	[3]									(65,867)	(60,220)	(54,242)
Capital Expenditures	[3]									1,598	681	23,459
Depreciation and amortization	[1],[3]									$ 11,443	$ 11,520	$ 10,798

[1] Depreciation and amortization is included in income from construction operations.

[2] As of December 31,(in thousands)20172016Civil$ 2,452,108$ 2,152,123Building 909,207 917,317Specialty Contractors 767,807 813,851Corporate and other(a) 135,001 155,329Total assets$ 4,264,123$ 4,038,620(a) Consists principally of cash, equipment, tax-related assets and insurance-related assets, offset by the elimination of assets related to intersegment revenue.

[3] Reportable SegmentsSpecialtySegmentConsolidated(in thousands)CivilBuildingContractorsTotalCorporateTotalYear ended December 31, 2017Total revenue$ 1,856,164$ 1,982,857$ 1,213,708$ 5,052,729$ -$ 5,052,729Elimination of intersegment revenue (253,989) (41,532) - (295,521) - (295,521)Revenue from external customers$ 1,602,175$ 1,941,325$ 1,213,708$ 4,757,208$ -$ 4,757,208Income from construction operations$ 192,207$ 34,199$ 18,938$ 245,344$ (65,867)(a)$ 179,477Capital expenditures$ 27,694$ 267$ 721$ 28,682$ 1,598$ 30,280Depreciation and amortization(b)$ 33,767$ 2,021$ 4,699$ 40,487$ 11,443$ 51,930Year ended December 31, 2016Total revenue$ 1,830,857$ 2,146,747$ 1,234,272$ 5,211,876$ -$ 5,211,876Elimination of intersegment revenue (161,894) (76,906) - (238,800) - (238,800)Revenue from external customers$ 1,668,963$ 2,069,841$ 1,234,272$ 4,973,076$ -$ 4,973,076Income from construction operations$ 172,668$ 51,564$ 37,908$ 262,140$ (60,220)(a)$ 201,920Capital expenditures$ 13,541$ 516$ 1,005$ 15,062$ 681$ 15,743Depreciation and amortization(b)$ 48,561$ 2,186$ 5,035$ 55,782$ 11,520$ 67,302Year ended December 31, 2015Total revenue$ 2,005,193$ 1,900,492$ 1,228,030$ 5,133,715$ -$ 5,133,715Elimination of intersegment revenue (115,286) (97,957) - (213,243) - (213,243)Revenue from external customers$ 1,889,907$ 1,802,535$ 1,228,030$ 4,920,472$ -$ 4,920,472Income from construction operations$ 145,213$ (1,240)$ 15,682$ 159,655$ (54,242)(a)$ 105,413Capital expenditures$ 8,383$ 2,877$ 1,193$ 12,453$ 23,459$ 35,912Depreciation and amortization(b)$ 22,601$ 2,728$ 5,507$ 30,836$ 10,798$ 41,634Consists primarily of corporate general and administrative expenses.