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Financial Instruments - Fair Value of Long Term Debt (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of long term debt $ 662 $ 749
Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of long term debt 658 634
Level 3    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of long term debt $ 15 $ 15