Revisions in Estimates (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) project	Sep. 30, 2016 USD ($) project	Sep. 30, 2017 USD ($) project	Sep. 30, 2016 USD ($) project
Change in Accounting Estimates
Impact From Affirmative Claim Recovery Estimate	$ 11.5	$ 1.2	$ 25.5	$ 21.4
Impact From Estimated Recovery of Backcharge Claims	0.6	3.3	3.6	10.9
Construction Segment
Change in Accounting Estimates
Change in Accounting Estimate Amount Considered Significant to Individual Project Gross Profit	$ 1.0	$ 1.0	$ 1.0	$ 1.0
Upward Estimate Change [Member] | Construction Segment
Change in Accounting Estimates
Number of Projects with Estimate Changes | project	2	1	5	2
Increase (Decrease) on Project Profitability	$ 2.2	$ 2.6	$ 6.0	$ 4.5
Upward Estimate Change [Member] | Construction Segment | Minimum
Change in Accounting Estimates
Range Of Effect On Gross Profit	1.0	2.6	1.0	1.8
Upward Estimate Change [Member] | Construction Segment | Maximum
Change in Accounting Estimates
Range Of Effect On Gross Profit	$ 1.2	$ 2.6	$ 1.6	$ 2.7
Upward Estimate Change [Member] | Large Project Construction [Member]
Change in Accounting Estimates
Number of Projects with Estimate Changes | project	0	3	0	7
Increase (Decrease) on Project Profitability	$ 0.0	$ 6.1	$ 0.0	$ 19.8
Upward Estimate Change [Member] | Large Project Construction [Member] | Minimum
Change in Accounting Estimates
Range Of Effect On Gross Profit	0.0	1.4	0.0	1.2
Upward Estimate Change [Member] | Large Project Construction [Member] | Maximum
Change in Accounting Estimates
Range Of Effect On Gross Profit	0.0	3.0	0.0	6.5
Net Estimate Change [Member] | Construction Segment
Change in Accounting Estimates
Increase (Decrease) on Project Profitability	(0.3)	0.7	(1.3)	(3.1)
Net Estimate Change [Member] | Large Project Construction [Member]
Change in Accounting Estimates
Increase (Decrease) on Project Profitability	$ (19.3)	$ 0.4	$ (58.1)	$ (6.0)
Downward Estimate Change [Member] | Construction Segment
Change in Accounting Estimates
Number of Projects with Estimate Changes | project	1	1	3	5
Increase (Decrease) on Project Profitability	$ (2.5)	$ (1.9)	$ (7.3)	$ (7.6)
Downward Estimate Change [Member] | Construction Segment | Minimum
Change in Accounting Estimates
Range Of Effect On Gross Profit	(2.5)	(1.9)	(1.1)	(1.1)
Downward Estimate Change [Member] | Construction Segment | Maximum
Change in Accounting Estimates
Range Of Effect On Gross Profit	$ (2.5)	$ (1.9)	$ (3.5)	$ (2.6)
Downward Estimate Change [Member] | Large Project Construction [Member]
Change in Accounting Estimates
Number of Projects with Estimate Changes | project	3	3	7	4
Increase (Decrease) on Project Profitability	$ (19.3)	$ (5.7)	$ (58.1)	$ (25.8)
Downward Estimate Change [Member] | Large Project Construction [Member] | Minimum
Change in Accounting Estimates
Range Of Effect On Gross Profit	(2.4)	(1.1)	(1.3)	(3.4)
Downward Estimate Change [Member] | Large Project Construction [Member] | Maximum
Change in Accounting Estimates
Range Of Effect On Gross Profit	(12.7)	(3.5)	(14.5)	(7.9)
Contracts Accounted for under Percentage of Completion [Member] | Large Project Construction [Member] | Noncontrolling Interest [Member]
Change in Accounting Estimates
Increase (Decrease) on Project Profitability	0.0	0.0	(2.1)	(3.5)
Estimated costs recorded in prior period
Change in Accounting Estimates
Impact From Affirmative Claim Recovery Estimate	0.5	1.0	0.4	5.9
Impact From Estimated Recovery of Backcharge Claims	0.0	2.1	1.6	7.5
Revisions to estimated total contract costs
Change in Accounting Estimates
Impact From Affirmative Claim Recovery Estimate	11.0	0.2	25.1	15.5
Impact From Estimated Recovery of Backcharge Claims	$ 0.6	$ 1.2	$ 2.0	3.4
Sales Revenue, Net
Change in Accounting Estimates
Misstatement in prior period filing				4.9
Gross Profit
Change in Accounting Estimates
Misstatement in prior period filing				$ 1.6