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Debt - Senior Notes and Contingent Convertible Senior Notes Narrative (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Long-Term Debt Instrument [Line Items]      
Cash paid to purchase debt $ 472 $ 0  
Long-term Debt, Gross 10,405   $ 10,692
Gain (Loss) on Repurchase of Debt Instrument 100    
Senior Notes [Member] | 3.25% Senior Notes due 2016 [Member]      
Long-Term Debt Instrument [Line Items]      
Cash paid to purchase debt $ 259    
Debt Instrument, Interest Rate, Stated Percentage 3.25%    
Debt Instrument, Repurchased Face Amount $ 122    
Debt Instrument, Repurchase Amount 115    
Long-term Debt, Gross $ 0   381
Senior Notes [Member] | 2.5% Contingent Convertible Senior Notes due 2037 [Member]      
Long-Term Debt Instrument [Line Items]      
Debt Instrument, Interest Rate, Stated Percentage 2.50%    
Long-term Debt, Gross $ 846   1,027
Senior Notes [Member] | 6.5% Senior Notes Due 2017 [Member]      
Long-Term Debt Instrument [Line Items]      
Debt Instrument, Interest Rate, Stated Percentage 6.50%    
Debt Instrument, Repurchased Face Amount $ 59    
Debt Instrument, Repurchase Amount 36    
Long-term Debt, Gross 379   $ 453
Senior Notes [Member] | 6.5% Senior Notes Due 2017 [Member] | Convertible Debt [Member]      
Long-Term Debt Instrument [Line Items]      
Debt Instrument, Repurchase Amount $ 15    
Senior Notes [Member] | Common Stock [Member] | 6.5% Senior Notes Due 2017 [Member]      
Long-Term Debt Instrument [Line Items]      
Debt Conversion, Converted Instrument, Shares Issued 2,555,979    
Convertible Debt [Member] | 2.5% Contingent Convertible Senior Notes due 2037 [Member]      
Long-Term Debt Instrument [Line Items]      
Debt Instrument, Interest Rate, Stated Percentage 2.50%    
Debt Instrument, Repurchased Face Amount $ 118    
Debt Instrument, Repurchase Amount 63    
Convertible Debt [Member] | 2.5% Contingent Convertible Senior Notes due 2037 [Member] | Convertible Debt [Member]      
Long-Term Debt Instrument [Line Items]      
Debt Instrument, Repurchase Amount $ 90    
Convertible Debt [Member] | Common Stock [Member] | 2.5% Contingent Convertible Senior Notes due 2037 [Member]      
Long-Term Debt Instrument [Line Items]      
Debt Conversion, Converted Instrument, Shares Issued 14,699,368