Revenue (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating Income (Loss)	$ (925)	$ 37,017
Civil [Member]
Operating Income (Loss)	2,839	31,888
Revenue, Remaining Performance Obligation	4,500,000
Civil [Member] | Scenario Unfavorable Adjustment [Member]
Operating Income (Loss)	17,800
Building [Member]
Operating Income (Loss)	6,425	5,242
Revenue, Remaining Performance Obligation	2,200,000
Specialty Contractors [Member]
Operating Income (Loss)	7,235	$ 14,762
Revenue, Remaining Performance Obligation	$ 1,800,000
Minimum [Member[ | Civil [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	3 years
Minimum [Member[ | Building And Specialty Contractors [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Maximum [Member] | Civil [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	5 years
Maximum [Member] | Building And Specialty Contractors [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	3 years