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Fair Value of Financial Instruments (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Number of days from period end 2 days  
Other Current Assets    
Assets    
Available-for-sale Securities, Current $ 4 $ 69
Other Assets    
Assets    
Available-for-sale Securities, Noncurrent 4 37
Long-term debt    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt, Fair Value 12,008 12,457
Carrying value    
Assets    
Debt, Long-term and Short-term, Combined Amount 10,894 10,942
Fair Value, Inputs, Level 1 [Member]    
Assets    
Fair value of investments and fixed income securities 0 0
Fair Value, Inputs, Level 1 [Member] | Long-term debt    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt, Fair Value 9,862 3,285
Level 2 | Long-term debt    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt, Fair Value 2,146 9,172
Level 3    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt, Fair Value 0 0
Assets    
Fair value of investments and fixed income securities $ 0 $ 175