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Condensed Consolidating Financial Information (Narrative) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument    
Long-term debt $ 16,633 $ 16,704
Recourse Debt    
Debt Instrument    
Long-term debt 7,182 $ 7,795
Senior Notes | Recourse Debt    
Debt Instrument    
Long-term debt $ 4,800