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Accounting for Asset Retirement Obligations Contingencies - Gypstack (Details) - USD ($)
$ in Millions
Aug. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Loss Contingencies [Line Items]        
Discounted asset retirement obligation   $ 1,160.1 $ 859.3 $ 849.9
Amount deposited by Mosaic into the RCRA Trusts $ 630.0      
Surety Bonds Outstanding Delivered To EPA   233.7    
Unfavorable Regulatory Action [Member]        
Loss Contingencies [Line Items]        
Discounted asset retirement obligation   578.4 529.7  
2015 Consent Decrees With EPA [Member] | Unfavorable Regulatory Action [Member]        
Loss Contingencies [Line Items]        
Payment related to loss contingency liability $ 200.0      
Letters of Credit to be issued to support financial assurance obligations under the Florida Consent     50.0  
Asset retirement obligations, undiscounted   1,500.0    
Discounted asset retirement obligation   $ 457.1    
Bonnie Facility Trust [Member]        
Loss Contingencies [Line Items]        
Assets held-in-trust, current     $ 21.0