Significant Accounting Policies (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Change in Accounting Estimate [Line Items]
Income from construction operations	$ 70,354,000	$ 58,094,000	$ 177,294,000	$ 133,647,000
Net income	35,730,000	23,759,000	80,214,000	54,037,000
Diluted earnings per common share (in dollars per share)	$ 0.73	$ 0.49	$ 1.64	$ 1.11
Increases in the Estimated Recovery Projected for Building Segment Projects [Member]
Change in Accounting Estimate [Line Items]
Favorable increase in results of operations	14,100,000
Income from construction operations		9,700,000		11,500,000
Net income		6,200,000		7,200,000
Diluted earnings per common share (in dollars per share)		$ 0.13		$ 0.15
Increases in Estimated Recovery Civil Segments Project [Member]
Change in Accounting Estimate [Line Items]
Number of projects impacted by change in the estimated recoveries			2
Favorable increase in results of operations			25,900,000
Unfavorable decrease in results of operations			9,400,000
Income from construction operations	15,800,000		30,600,000
Net income	$ 9,500,000		$ 18,300,000
Diluted earnings per common share (in dollars per share)	$ 0.19		$ 0.37