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Debt - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Long-Term Debt Instrument [Line Items]            
Cash paid to purchase debt         $ 0 $ 1,746
Gains (losses) on restructuring of debt $ 0 $ 191 $ 184 $ 191 0 $ 184
Debt instrument, face amount 9,706   9,981   9,706  
Senior Notes, Sr. Secured Notes, Contingent Convertible Notes [Member]            
Long-Term Debt Instrument [Line Items]            
Debt instrument, repurchased face amount     1,604      
Cash paid to purchase debt     1,746      
Revolving credit facility [Member]            
Long-Term Debt Instrument [Line Items]            
Line of credit facility, current borrowing capacity 3,800       3,800  
Letters of credit outstanding, amount 183       183  
Revolving credit facility [Member] | Line of Credit [Member]            
Long-Term Debt Instrument [Line Items]            
Debt instrument, face amount $ 506   $ 781   $ 506