Revenue (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Net revenue recognized related to performance obligations satisfies (or partially satisfied) in prior periods	$ 77.0	$ 177.5	$ 19.4
Alaskan Way Viaduct Matter
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Net revenue recognized related to performance obligations satisfies (or partially satisfied) in prior periods		123.9
Civil
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance obligation amount	$ 4,800.0	5,200.0
Civil | Minimum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation period range	3 years
Civil | Maximum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation period range	5 years
Building
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance obligation amount	$ 1,500.0	2,200.0
Specialty Contractors
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance obligation amount	$ 1,800.0	$ 2,200.0
Building And Specialty Contractors | Minimum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation period range	1 year
Building And Specialty Contractors | Maximum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation period range	3 years