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Financial Instruments and Fair Value Measurements - Carrying Values and Estimated Fair Values of Financial Instruments (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt $ 13,128 $ 13,268
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt fair value 13,230 13,787
Level 1 | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt fair value 0 0
Level 2 | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt fair value 9,736 10,115
Level 3 | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt fair value $ 3,494 $ 3,672