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Employee Retirement Benefits - Contributions to Multiemployer Plans (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
plan
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Multiemployer Plans [Line Items]      
Number of other multiemployer plans | plan 6    
Multiemployer Plans, Pension      
Multiemployer Plans [Line Items]      
Contributions $ 1.7 $ 1.5 $ 1.4
Multiemployer Plans, Pension | Midwest Operating Engineers      
Multiemployer Plans [Line Items]      
Contributions 0.8 0.7 0.7
Multiemployer Plans, Pension | Other Plans      
Multiemployer Plans [Line Items]      
Contributions $ 0.9 $ 0.8 $ 0.7