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Financial Instruments and Other Guarantees - Textuals (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Mar. 31, 2019
Dec. 31, 2018
Guarantee Obligations [Line Items]        
Surety Bonds outstanding $ 1,571.3   $ 1,571.3  
Letters of credit outstanding, amount 239.6   239.6  
Asset retirement obligations 691.8   691.8 $ 686.4
Net interest expense 35.8 $ 36.3    
Net impact on collateral requirements 0.0 214.0    
Provision for North Goonyella equipment loss 24.7 0.0   $ 66.4
Proceeds from Release of Collateral, Short-term and Long-term   254.1    
Secured Debt | Accounts Receivable Securitization Program, April 3, 2020        
Guarantee Obligations [Line Items]        
Line of Credit Facility, Maximum Borrowing Capacity 250.0   250.0  
Secured Debt | Accounts Receivable Securitization Program, April 1, 2022 [Member]        
Guarantee Obligations [Line Items]        
Letters of credit outstanding, amount 0.0   0.0  
Letters of credit drawn on, amount 131.7   131.7  
Cash collateral required 0.0   0.0  
Net interest expense 1.1 $ 1.3    
U.S Mining        
Guarantee Obligations [Line Items]        
Surety Bonds outstanding 1,358.9   1,358.9  
Letters of credit outstanding, amount 147.3   147.3  
Asset retirement obligations $ 755.7   755.7  
Leased Equipment        
Guarantee Obligations [Line Items]        
Provision for North Goonyella equipment loss     $ 50.7