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Commitments (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2018
USD ($)
t
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity      
2019 $ 2,586.5    
2020 588.9    
2021 495.7    
2022 375.5    
2023 261.1    
Subsequent years 1,437.9    
Total 5,745.6    
A schedule of future minimum lease payments under non-cancelable operating leases follows:      
2019 97.5    
2020 76.8    
2021 54.7    
2022 36.6    
2023 28.1    
Subsequent years 30.9    
Total 324.6    
Rental expense for the fiscal period was as follows:      
Rental expense for the fiscal period 270.3 $ 114.0 $ 111.0
Purchases made for the fiscal period were as follows:      
Purchases made under long-term commitments during the reporting period 2,000.0 $ 1,900.0 $ 1,600.0
Surety Bonds Outstanding      
Total amount of surety bonds outstanding 497.7    
Surety bonds outstanding for mining reclamation obligations 203.3    
Surety Bonds Outstanding Delivered To EPA 233.7    
Surety bonds outstanding for other than mining reclamation obligations $ 60.7    
Inventories At Price Tied To Natural Gas      
Long-term Purchase Commitment      
Long-term Purchase Commitment, Minimum Quantity Required | t 545,000    
Long Term Purchase Commitment Maximum Quantity Required | t 725,000