Commitments and Contingencies (Narrative) (Details) - USD ($)			12 Months Ended
	Feb. 28, 2017	Jan. 14, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 31, 2017
Commitments And Contingencies [Line Items]
Outstanding performance bonds			$ 1,389,239,000
Revenue value remaining from outstanding performance bonds			619,426,000
Proceeds from Legal Settlements		$ 5,309,000
Rent expense under long term operating lease arrangements			21,160,000	$ 26,664,000	$ 20,006,000
Surety Bond [Member]
Commitments And Contingencies [Line Items]
Aggregate demolition surety performance bond	$ 20,000,000
Letter of Credit [Member]
Commitments And Contingencies [Line Items]
Line of credit facility, maximum borrowing capacity						$ 20,881,000
Environmental and Infrastructure Segment [Member]
Commitments And Contingencies [Line Items]
Outstanding performance bonds			78,314,000
Minimum [Member]
Commitments And Contingencies [Line Items]
Bids bond range			$ 1,000,000
Warranty periods			1 year
Loss contingency, Estimate of possible loss			$ 200,000
Maximum [Member]
Commitments And Contingencies [Line Items]
Bids bond range			$ 10,000,000
Warranty periods			3 years
Loss contingency, Estimate of possible loss			$ 250,000