Goodwill and Other Intangible Assets (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill	$ 585,006	$ 585,006	$ 585,000
Amortization expense	3,543	3,543	3,715
Finite-Lived intangible assets
2018	3,500
2019	3,500
2020	3,500
2021	3,500
2022	2,600
Civil
Goodwill	415,300	415,300	415,300
Accumulated Impairment	76,700	76,700	76,700
Building
Goodwill	13,500	13,500	13,500
Accumulated Impairment	411,300	411,300	411,300
Specialty Contractors
Goodwill	156,200	156,200	$ 156,200
Trade Names (Non-amortizable)
Indefinite-lived intangible assets
Indefinite-lived intangible assets, carrying value	50,400	50,400
Trade Names (Non-amortizable)
Finite-Lived intangible assets
Finite-lived intangible assets, gross carrying value	51,100	51,100
Accumulated Amortization	$ 16,300	13,800
Weighted Average Amortization Period	20 years
Customer relationships
Finite-Lived intangible assets
Finite-lived intangible assets, gross carrying value	$ 23,200	23,200
Accumulated Amortization	$ 18,900	$ 17,900
Weighted Average Amortization Period	12 years