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Debt (Narrative) (Details) - USD ($)
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Debt Instrument [Line Items]      
Short-term borrowings $ 0   $ 0
Loss on early extinguishment of debt 0 $ 228,000,000  
Debt tender premium and fees   226,000,000  
Write-off of unamortized debt discount   $ 2,000,000  
Reported Value Measurement [Member]      
Debt Instrument [Line Items]      
Fair value of long term debt 12,900,000,000   12,800,000,000
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value Measurement [Member]      
Debt Instrument [Line Items]      
Fair value of long term debt 15,500,000,000   $ 14,500,000,000
Senior Notes [Member]      
Debt Instrument [Line Items]      
Interest rate   9.70%  
Senior Notes [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Notes purchased   $ 793,000,000  
Senior Notes [Member] | Notes Issued, 2008 and 2009 [Member] | Subject to Credit Rating Step Up [Member]      
Debt Instrument [Line Items]      
Aggregate principal amount $ 3,400,000,000