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Backlog (Details) - Construction
$ in Millions
12 Months Ended
Dec. 31, 2016
USD ($)
contract
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Purchase Commitment, Excluding Long-term Commitment [Line Items]      
Backlog $ 503.5 $ 380.8 $ 357.0
Contracts or Purchase Orders      
Purchase Commitment, Excluding Long-term Commitment [Line Items]      
Backlog 441.1 252.7 305.3
Letters of Intent      
Purchase Commitment, Excluding Long-term Commitment [Line Items]      
Backlog 62.4 $ 128.1 $ 51.7
Five Contracts, Letters of Intent, Notices to Proceed or Purchase Orders      
Purchase Commitment, Excluding Long-term Commitment [Line Items]      
Backlog $ 296.4    
Backlog percentage 58.90%    
Number of contracts attributable to backlog | contract 5