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Financial Instruments and Other Guarantees - Textuals (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Guarantee Obligations [Line Items]          
Surety Bonds outstanding $ 1,599.3   $ 1,599.3    
Letters of credit outstanding, amount 200.5   200.5    
Asset retirement obligations 696.2   696.2   $ 686.4
Net interest expense 35.4 $ 38.2 107.2 $ 112.8  
Net impact on collateral requirements     0.0 323.1  
Provision for North Goonyella equipment loss 0.0 49.3 24.7 49.3 66.4
Proceeds from Release of Collateral, Short-term and Long-term       363.2  
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 issued, amount 132.7   132.7    
Line of Credit Facility, Maximum Borrowing Capacity 250.0   250.0    
Cash collateral required 0.0   0.0   $ 0.0
Net interest expense 1.0 $ 1.7 3.6 $ 5.5  
U.S Mining          
Guarantee Obligations [Line Items]          
Surety Bonds outstanding 1,394.9   1,394.9    
Letters of credit outstanding, amount 106.1   106.1    
Asset retirement obligations $ 760.0   $ 760.0