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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, 2017
Dec. 31, 2016
Financial Assets [Abstract]    
Mortgage loans $ 839 $ 591
Financial Liabilities [Abstract]    
Short term debt 150 0
Long term debt 2,708 2,710
Estimated Fair Value    
Financial Assets [Abstract]    
Mortgage loans 844 594
Note receivable 46  
Financial Liabilities [Abstract]    
Short term debt 150  
Long term debt 2,896 2,952
Carrying Amount    
Financial Assets [Abstract]    
Mortgage loans 839 591
Note receivable 46  
Financial Liabilities [Abstract]    
Short term debt 150  
Long term debt 2,708 2,710
Level 1 | Estimated Fair Value    
Financial Assets [Abstract]    
Mortgage loans 0 0
Note receivable 0  
Financial Liabilities [Abstract]    
Short term debt 0  
Long term debt 0 0
Level 2 | Estimated Fair Value    
Financial Assets [Abstract]    
Mortgage loans 0 0
Note receivable 0  
Financial Liabilities [Abstract]    
Short term debt 150  
Long term debt 2,896 2,952
Level 3 | Estimated Fair Value    
Financial Assets [Abstract]    
Mortgage loans 844 594
Note receivable 46  
Financial Liabilities [Abstract]    
Short term debt 0  
Long term debt $ 0 $ 0