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Income Taxes (Details Textuals) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Valuation Allowance [Line Items]        
Operating Income (Loss)   $ 6,300.0    
Income Taxes (Textuals) [Abstract]        
Alternative minimum tax credits   264.3    
General business credits   119.4    
Deferred Tax Assets, Operating Loss Carryforwards, Domestic   2,340.4    
State net operating loss carryforwards   127.9    
Deferred Tax Assets, Charitable Contribution Carryforwards   1.3    
Foreign loss carryforwards included in Company's tax credits and loss carryforwards   1,102.2    
Valuation allowance reserve for U.S. capital losses, state NOLs, foreign NOLs and certain foreign deferred tax assets $ (1,614.1) (4,037.5) $ (1,614.1)  
Minerals resource rent tax, net of federal tax   0.0 0.0 $ 16.1
Change in unrecognized tax benefit   2.8    
Net unrecognized tax benefits 19.6 20.1 19.6  
Accrued interest related to unrecognized tax benefits included in income tax provision   0.4 2.1 $ 8.0
Accrued interest related to uncertain tax positions 0.4 2.4 $ 0.4  
Australia Deferred Tax Assets [Member]        
Income Taxes (Textuals) [Abstract]        
Valuation Allowance, Deferred Tax Asset, Change in Amount   82.1    
US Deferred Tax Assets [Member]        
Income Taxes (Textuals) [Abstract]        
Capital loss   60.8    
Valuation Allowance, Deferred Tax Asset, Change in Amount $ (177.0) $ 2,342.9