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Fair Value Measurements Fair Values And Related Carrying Values of Financial Instruments (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Notes receivable, Fair Value $ 682 $ 803
Debt, excluding capital lease obligations, Fair Value (11,923) (11,209)
Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Notes receivable, Fair Value 682 803
Debt, excluding capital lease obligations, Fair Value (11,823) (11,078)
Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt, excluding capital lease obligations, Fair Value (100) (131)
Carrying Amount [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Notes receivable, net 677 807
Debt, excluding capital lease obligations, Carrying Amount $ (10,380) $ (9,815)