Revenue (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating Income (Loss)	$ 54,815	$ 34,045	$ 53,890	$ 71,062
Civil [Member]
Operating Income (Loss)	49,439	58,144	52,278	90,032
Revenue, Remaining Performance Obligation	4,300,000		4,300,000
Civil [Member] | Scenario Unfavorable Adjustment [Member]
Operating Income (Loss)			17,800
Building [Member]
Operating Income (Loss)	12,536	5,736	18,961	10,977
Revenue, Remaining Performance Obligation	2,000,000		2,000,000
Specialty Contractors [Member]
Operating Income (Loss)	7,454	$ (14,007)	14,689	$ 755
Revenue, Remaining Performance Obligation	$ 1,600,000		$ 1,600,000
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