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Fair Value Measurements (Details) - USD ($)
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Transfer of Level 1 to Level 2 assets $ 0  
Transfer of Level 2 to Level 1 assets 0  
Transfer of Level 1 to Level 2 liabilities 0  
Transfer of Level 2 to Level 1 liabilities 0  
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Fair value of investments 53,000,000 $ 51,000,000
Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Fair value of assets or liabilities 0 0
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Fair value of assets or liabilities 0 0
Carrying Amount [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Long-term debt, including the current portion 4,984,000,000 4,753,000,000
Fair Value [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Long-term debt, including the current portion $ 5,131,000,000 $ 5,034,000,000