Business Segments (Reportable Segments) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenue	$ 1,178,222	$ 1,442,091	$ 3,605,060	$ 3,969,247
Income (loss) from construction operations	52,093	83,021	170,550	187,944
Capital expenditures	8,302	12,010	27,162	43,396
Depreciation and amortization	30,061	30,858	92,196	79,634
Operating Segments
Segment Reporting Information [Line Items]
Revenue	1,290,878	1,598,238	3,996,366	4,335,358
Income (loss) from construction operations	67,871	95,752	213,061	225,467
Capital expenditures	8,068	11,658	26,536	42,727
Depreciation and amortization	27,427	28,080	84,025	71,314
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue	(112,656)	(156,147)	(391,306)	(366,111)
Corporate
Segment Reporting Information [Line Items]
Income (loss) from construction operations	(15,778)	(12,731)	(42,511)	(37,523)
Capital expenditures	234	352	626	669
Depreciation and amortization	2,634	2,778	8,171	8,320
Civil
Segment Reporting Information [Line Items]
Revenue	546,218	611,996	1,577,145	1,667,601
Civil | Favorable Adjustments Reflecting Improved Profitability on Mass-Transit Project
Segment Reporting Information [Line Items]
Favorable arbitration			18,100
Favorable arbitration, after tax			$ 13,000
Favorable arbitration, after tax, diluted (in dollars per share)			$ 0.25
Civil | Operating Segments
Segment Reporting Information [Line Items]
Revenue	624,549	723,324	$ 1,850,748	1,948,095
Income (loss) from construction operations	62,555	70,237	187,733	181,756
Capital expenditures	7,847	10,996	26,027	41,139
Depreciation and amortization	26,234	26,659	80,125	67,050
Civil | Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue	(78,331)	(111,328)	(273,603)	(280,494)
Building
Segment Reporting Information [Line Items]
Revenue	360,941	508,140	1,150,834	1,462,925
Building | Operating Segments
Segment Reporting Information [Line Items]
Revenue	395,013	552,823	1,267,984	1,548,223
Income (loss) from construction operations	10,786	15,815	19,514	37,120
Capital expenditures	87	438	211	636
Depreciation and amortization	416	419	1,272	1,274
Building | Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue	(34,072)	(44,683)	(117,150)	(85,298)
Specialty Contractors
Segment Reporting Information [Line Items]
Revenue	271,063	321,955	877,081	838,721
Specialty Contractors | Favorable Legal Ruling Pertaining to Dispute in Specialty Contractors Segment
Segment Reporting Information [Line Items]
Favorable arbitration		19,600		19,600
Favorable arbitration, after tax		$ 14,100		$ 14,100
Favorable arbitration, after tax, diluted (in dollars per share)		$ 0.28		$ 0.28
Specialty Contractors | Unfavorable Legal Ruling Pertaining to Mechanical Project in California
Segment Reporting Information [Line Items]
Adverse arbitration, loss in period		$ 15,200		$ 15,200
Adverse arbitration, loss in period, after tax		$ 10,900		$ 10,900
Adverse arbitration, loss in period, after tax, diluted (in dollars per share)		$ 0.21		$ 0.21
Specialty Contractors | Favorable Legal Ruling Pertaining to Electrical Project in New York
Segment Reporting Information [Line Items]
Favorable arbitration			20,100
Favorable arbitration, after tax			$ 14,600
Favorable arbitration, after tax, diluted (in dollars per share)			$ 0.28
Specialty Contractors | Unfavorable Adjustment Due to Changes in Estimate on Electrical Project in New York
Segment Reporting Information [Line Items]
Adverse arbitration, loss in period			$ (14,500)
Adverse arbitration, loss in period, after tax			$ (10,500)
Adverse arbitration, loss in period, after tax, diluted (in dollars per share)			$ (0.21)
Specialty Contractors | Adverse Arbitration Ruling Pertaining to Electrical Project in New York
Segment Reporting Information [Line Items]
Adverse arbitration, loss in period				$ 13,200
Adverse arbitration, loss in period, after tax				$ 9,500
Adverse arbitration, loss in period, after tax, diluted (in dollars per share)				$ 0.19
Specialty Contractors | Operating Segments
Segment Reporting Information [Line Items]
Revenue	271,316	$ 322,091	$ 877,634	$ 839,040
Income (loss) from construction operations	(5,470)	9,700	5,814	6,591
Capital expenditures	134	224	298	952
Depreciation and amortization	777	1,002	2,628	2,990
Specialty Contractors | Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (253)	$ (136)	$ (553)	$ (319)