Business Segments (Reportable Segments) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Segments
Revenue	$ 1,349,516	$ 1,442,091	$ 1,276,427	$ 1,250,729	$ 1,177,725	$ 1,189,345	$ 1,125,275	$ 958,487	$ 5,318,763	$ 4,450,832	$ 4,454,662
Income from construction operations	$ 74,400	$ 83,021	$ 57,696	$ 47,227	$ (94,146)	$ 47,943	$ (341,717)	$ 22,913	262,344	(365,007)	191,876
Capital expenditures									54,781	84,196	77,069
Depreciation and amortization									107,034	65,044	47,267
Goodwill impairment charge									0	379,863	0
Goodwill impairment charge, after tax										$ 330,500
Goodwill impairment charge, after tax, diluted (in dollars per share)										$ 6.58
Operating Segments
Business Segments
Revenue									5,814,687	$ 4,748,588	4,684,004
Income from construction operations									316,196	(299,819)	255,625
Capital expenditures									53,839	83,362	76,298
Depreciation and amortization									95,936	53,975	35,999
Corporate, Non-Segment
Business Segments
Income from construction operations									(53,852)	(65,188)	(63,749)
Capital expenditures									942	834	771
Depreciation and amortization									11,098	11,069	11,268
Intersegment Eliminations
Business Segments
Revenue									(495,924)	(297,756)	(229,342)
Civil
Business Segments
Revenue									2,199,899	1,779,352	1,586,093
Loss contingency										37,800
Loss contingency, after tax										$ 27,100
Loss contingency, after tax, diluted (in dollars per share)										$ 0.54
Goodwill impairment charge										$ 210,215
Civil | Adverse Arbitration Ruling Pertaining To Subcontract Project In New York
Business Segments
Loss contingency											17,800
Loss contingency, after tax											$ 12,800
Loss contingency, after tax, diluted (in dollars per share)											$ 0.25
Civil | Alaskan Way Viaduct Matter
Business Segments
Loss contingency										166,800
Loss contingency, after tax										$ 119,400
Loss contingency, after tax, diluted (in dollars per share)										$ 2.38
Civil | Operating Segments
Business Segments
Revenue									2,565,210	$ 2,054,097	$ 1,810,232
Income from construction operations									245,835	(150,837)	168,256
Capital expenditures									51,044	82,156	73,866
Depreciation and amortization									90,250	47,905	29,685
Civil | Intersegment Eliminations
Business Segments
Revenue									(365,311)	(274,745)	(224,139)
Building
Business Segments
Revenue									1,984,641	1,742,040	1,861,699
Goodwill impairment charge										13,455
Building | Operating Segments
Business Segments
Revenue									2,114,459	1,764,753	1,866,902
Income from construction operations									53,158	23,655	43,939
Capital expenditures									878	518	1,655
Depreciation and amortization									1,703	1,934	1,956
Building | Intersegment Eliminations
Business Segments
Revenue									(129,818)	(22,713)	(5,203)
Specialty Contractors
Business Segments
Revenue									1,134,223	929,440	1,006,870
Goodwill impairment charge										156,193
Specialty Contractors | Unfavorable Legal Ruling Pertaining To Mechanical Project In California
Business Segments
Loss contingency									15,200
Loss contingency, after tax									$ 11,000
Loss contingency, after tax, diluted (in dollars per share)									$ 0.22
Specialty Contractors | Adverse Arbitration Ruling Pertaining To Electrical Project In New York
Business Segments
Loss contingency									$ 13,200
Loss contingency, after tax									$ 9,600
Loss contingency, after tax, diluted (in dollars per share)									$ 0.19
Specialty Contractors | Favorable Arbitration Decision
Business Segments
Loss contingency									$ 25,700
Loss contingency, after tax									$ 18,600
Loss contingency, after tax, diluted (in dollars per share)									$ 0.36
Specialty Contractors | Operating Segments
Business Segments
Revenue									$ 1,135,018	929,738	1,006,870
Income from construction operations									17,203	(172,637)	43,430
Capital expenditures									1,917	688	777
Depreciation and amortization									3,983	4,136	4,358
Specialty Contractors | Intersegment Eliminations
Business Segments
Revenue									$ (795)	$ (298)	$ 0