Business Segments (Narrative) (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Mar. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Sep. 30, 2016 USD ($) $ / shares	Jun. 30, 2016 USD ($) $ / shares	Mar. 31, 2016 USD ($) $ / shares	Dec. 31, 2017 USD ($) segment $ / shares	Dec. 31, 2016 USD ($) contract $ / shares	Dec. 31, 2015 USD ($) $ / shares
Segment Reporting Information [Line Items]
Number of reportable segments | segment									3
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
DILUTED EARNINGS PER COMMON SHARE | $ / shares	$ 1.60	$ 0.47	$ 0.59	$ 0.27	$ 0.60	$ 0.57	$ 0.43	$ 0.31	$ 2.92	$ 1.92	$ 0.91
Building
Segment Reporting Information [Line Items]
Income from construction operations									$ 34,199	$ 51,564	$ (1,240)
Building | Unfavorable Adjustments [Member]
Segment Reporting Information [Line Items]
Income from construction operations											$ (24,300)
DILUTED EARNINGS PER COMMON SHARE | $ / shares											$ (0.28)
Civil
Segment Reporting Information [Line Items]
Income from construction operations									192,207	172,668	$ 145,213
Civil | Unfavorable Adjustments [Member]
Segment Reporting Information [Line Items]
Income from construction operations											$ (23,900)
DILUTED EARNINGS PER COMMON SHARE | $ / shares											$ (0.28)
Civil Runway Reconstruction Project [Member] | Unfavorable Adjustments [Member]
Segment Reporting Information [Line Items]
Income from construction operations											$ 13,700
DILUTED EARNINGS PER COMMON SHARE | $ / shares											$ (0.16)
Specialty Contractors
Segment Reporting Information [Line Items]
Income from construction operations									$ 18,938	37,908	$ 15,682
Specialty Contractors | Unfavorable Adjustments [Member]
Segment Reporting Information [Line Items]
Income from construction operations										(13,800)
Five Star Electric [Member] | Unfavorable Adjustments [Member]
Segment Reporting Information [Line Items]
Income from construction operations											$ (45,600)
DILUTED EARNINGS PER COMMON SHARE | $ / shares											$ (0.53)
Five Star Electric [Member] | Specialty Contractors | Favorable Adjustments [Member]
Segment Reporting Information [Line Items]
Income from construction operations										$ 3,000
DILUTED EARNINGS PER COMMON SHARE | $ / shares										$ 0.04
Five Star Electric Subcontract [Member] | Specialty Contractors | Unfavorable Adjustments [Member]
Segment Reporting Information [Line Items]
DILUTED EARNINGS PER COMMON SHARE | $ / shares										$ (0.16)
Five Star Electric Subcontract [Member] | Specialty Contractors | Favorable Adjustments [Member]
Segment Reporting Information [Line Items]
Number of projects | contract										2
Income from construction operations										$ 14,000
DILUTED EARNINGS PER COMMON SHARE | $ / shares										$ 0.17