Business Segments (Narrative) (Details) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2019 USD ($) segment $ / shares	Sep. 30, 2018 USD ($) $ / shares	Dec. 31, 2018 segment
Segment Reporting Information [Line Items]
Number of reportable segments | segment			3		3
Operating Income (Loss)	$ 47,943	$ 47,306	$ (270,861)	$ 101,196
Net income (loss)	$ 26,721	$ 25,436	$ (283,996)	$ 42,390
DILUTED EARNINGS (LOSS) PER COMMON SHARE | $ / shares	$ 0.38	$ 0.42	$ (6.01)	$ 0.68
Scenario Unfavorable Adjustment [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)			$ 379,900
Net income (loss)			$ 329,500
DILUTED EARNINGS (LOSS) PER COMMON SHARE | $ / shares			$ 6.56
Building [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 7,580	$ 8,853	$ 6,903	$ 27,814
Civil [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	50,695	41,282	(72,032)	93,560
Civil [Member] | Scenario Unfavorable Adjustment [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)				17,800
Net income (loss)				$ 12,800
DILUTED EARNINGS (LOSS) PER COMMON SHARE | $ / shares				$ 0.25
Specialty Contractors [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	7,247	11,561	(160,036)	$ 26,250
Corporate [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ (17,579)	$ (14,390)	$ (45,696)	$ (46,428)