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Commitments and Contingencies (Details 2)
AUD in Millions, $ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2016
USD ($)
Sep. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2016
AUD
Dec. 31, 2015
USD ($)
Dec. 31, 2016
AUD
Sep. 30, 2016
AUD
Dec. 31, 2014
USD ($)
Loss Contingency [Abstract]                
Additional charge recorded as a result of the damages awarded to Eagle in arbitration     $ 15.6          
Undiscounted environmental clean-up liabilities, current $ 0.0   $ 0.0   $ 23.9      
Monto Coal Pty Limited [Member]                
Loss Contingency [Abstract]                
Ownership Percentage In Subsidiaries 51.00%   51.00%     51.00%    
Loss Contingency, Damages Sought, Value | AUD       AUD 15.6        
Loss contingency damages sought value max | AUD       1,800.0        
Eagle Mining [Member]                
Loss Contingencies [Line Items]                
Gain Related to Litigation Settlement         10.8      
Loss Contingency [Abstract]                
Loss Contingency Accrual               $ 23.4
Wilkie Creek [Member]                
Loss Contingency [Abstract]                
Loss Contingency, Damages Sought, Value | AUD       AUD 113.1        
Loss Contingency, Loss in Period         9.7      
Settlement charges total $ 9.9   $ 9.9     AUD 13.0 AUD 13.0  
Gold Fields [Member]                
Loss Contingency [Abstract]                
Undiscounted environmental clean-up liabilities, total $ 62.8   $ 62.8   $ 66.9      
Prarie State Energy Campus [Member]                
Loss Contingency [Abstract]                
Undivided Interest Percent Of New Electricity Generation Project 5.06%   5.06%     5.06%    
Senior Notes [Member]                
Loss Contingencies [Line Items]                
Face amount of senior notes $ 1,650.0   $ 1,650.0          
Bankruptcy unliquidated, general unsecured claim [Member] | Blue Tee [Member]                
Loss Contingency [Abstract]                
Bankruptcy Claims, Amount of Claims Filed     65.6          
Bankruptcy additional unliquidated, in an unknown amount [Member] | Blue Tee [Member]                
Loss Contingency [Abstract]                
Bankruptcy Claims, Amount of Claims Filed   $ 150.0            
Bankruptcy unliquidated, secured and general unsecured [Member] | Gold Fields [Member]                
Loss Contingency [Abstract]                
Bankruptcy Claims, Amount of Claims Filed     2,700.0          
Bankruptcy further contingent claims, at known and unknown sites [Member] | Blue Tee [Member]                
Loss Contingency [Abstract]                
Bankruptcy Claims, Amount of Claims Filed $ 1,200.0   $ 500.0