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Fair Value (Carrying amount and estimated fair value of financial instrument assets and liabilities which are not measured at fair value) (Details) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Financial Assets [Abstract]        
Notes receivable for the issuance of common stock $ 0 $ 1 $ 23 $ 21
Mortgage loans 678 588    
Financial Liabilities [Abstract]        
Short term debt 350 0    
Long term debt 2,212 2,559    
Carrying Amount [Member]        
Financial Assets [Abstract]        
Notes receivable for the issuance of common stock   1    
Mortgage loans 678 588    
Financial Liabilities [Abstract]        
Short term debt 350      
Long term debt 2,212 2,559    
Estimated Fair Value [Member]        
Financial Assets [Abstract]        
Notes receivable for the issuance of common stock   1    
Mortgage loans 688 608    
Financial Liabilities [Abstract]        
Short term debt 360      
Long term debt 2,433 2,883    
Level 1 [Member] | Estimated Fair Value [Member]        
Financial Assets [Abstract]        
Notes receivable for the issuance of common stock   0    
Mortgage loans 0 0    
Financial Liabilities [Abstract]        
Short term debt 0      
Long term debt 0 0    
Level 2 [Member] | Estimated Fair Value [Member]        
Financial Assets [Abstract]        
Notes receivable for the issuance of common stock   0    
Mortgage loans 0 0    
Financial Liabilities [Abstract]        
Short term debt 360      
Long term debt 2,433 2,883    
Level 3 [Member] | Estimated Fair Value [Member]        
Financial Assets [Abstract]        
Notes receivable for the issuance of common stock   1    
Mortgage loans 688 608    
Financial Liabilities [Abstract]        
Short term debt 0      
Long term debt $ 0 $ 0