Business Segments (Narrative) (Details) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2018 USD ($) segment $ / shares	Sep. 30, 2017 USD ($) $ / shares
Segment Reporting Information [Line Items]
Number of reportable segments | segment			3
Income from construction operations	$ 47,306	$ 49,072	$ 101,196	$ 120,134
After-tax impact	$ 25,436	$ 25,300	$ 42,390	$ 71,697
DILUTED EARNINGS PER COMMON SHARE | $ / shares	$ 0.42	$ 0.47	$ 0.68	$ 1.33
Building [Member]
Segment Reporting Information [Line Items]
Income from construction operations	$ 8,853	$ 14,058	$ 27,814	$ 25,035
Civil [Member]
Segment Reporting Information [Line Items]
Income from construction operations	41,282	38,144	93,560	128,176
Civil [Member] | Scenario Unfavorable Adjustment [Member]
Segment Reporting Information [Line Items]
Income from construction operations			17,800
After-tax impact			$ 12,800
DILUTED EARNINGS PER COMMON SHARE | $ / shares			$ 0.25
Specialty Contractors [Member]
Segment Reporting Information [Line Items]
Income from construction operations	11,561	14,575	$ 26,250	15,330
Corporate [Member]
Segment Reporting Information [Line Items]
Income from construction operations	$ (14,390)	$ (17,705)	$ (46,428)	$ (48,407)