Revenue (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Net revenue recognized related to performance obligations satisfies (or partially satisfied) in prior periods	$ 19.3
Civil
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations revenue amount	$ 4,800.0	$ 5,200.0
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	$ 1,500.0	1,900.0
Specialty Contractors
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations revenue amount	$ 1,700.0	$ 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