Revenue (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Net revenue recognized related to performance obligations satisfies (or partially satisfied) in prior periods	$ 30.4	$ 13.8	$ 71.3	$ 46.3
Civil
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations revenue amount	5,100.0	5,300.0	$ 5,100.0	5,300.0
Civil | Minimum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations revenue period			3 years
Civil | Maximum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations revenue period			5 years
Building
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations revenue amount	1,900.0	1,600.0	$ 1,900.0	1,600.0
Specialty Contractors
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations revenue amount	$ 1,900.0	$ 2,200.0	$ 1,900.0	$ 2,200.0
Building And Specialty Contractors | Minimum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations revenue period			1 year
Building And Specialty Contractors | Maximum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations revenue period			3 years