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Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Revenue Recognition [Line Items]          
Change in contract estimates, (favorable) unfavorable impact on operating results, percent (12.80%) (5.00%) (10.80%) (5.00%)  
Revenues recognized related to change orders and claims $ 181.4   $ 181.4   $ 141.2
Percent of total revenues recognized associated with revenue recognition method 43.90% 48.40% 43.90% 48.20%  
Revenue, related to performance obligation satisfied in previous periods     $ 105.0    
Revenue recognized related to amounts in contract liabilities outstanding at the beginning of period     332.6    
Current retainage balances $ 283.0   283.0   306.3
Non-current retainage balances 119.2   119.2   88.2
Unbilled receivables 643.0   643.0   472.3
Accounts Payable and Accrued Expenses          
Revenue Recognition [Line Items]          
Unearned revenues, current portion 42.6   42.6   $ 53.6
Domestic Communications Project          
Revenue Recognition [Line Items]          
Change in contract estimates, (favorable) unfavorable impact on operating results, amount     14.8    
Contract value $ 109.4   $ 109.4    
Percentage of project completion 51.00%   51.00%    
Pipeline Transmission Project - Canada - Project One          
Revenue Recognition [Line Items]          
Change in contract estimates, (favorable) unfavorable impact on operating results, amount       $ 14.1  
Contract value $ 115.6   $ 115.6