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Accounting for Asset Retirement Obligations Other Commitments (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Other Commitments [Line Items]      
Surety Bonds Outstanding Delivered To EPA $ 233.7    
Restricted cash 15.8 $ 32.6 $ 31.3
Asset retirement obligations 1,023.8 761.2  
Bonnie Facility Trust [Member]      
Other Commitments [Line Items]      
Assets held-in-trust, current   21.0  
Plant City and Bonnie Facilities [Member]      
Other Commitments [Line Items]      
Asset retirement obligations $ 109.2 $ 97.7