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Fair Value Measurements (Details) - USD ($)
Sep. 30, 2017
Dec. 31, 2016
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 50,000,000 $ 59,000,000
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 4,816,000,000 4,865,000,000
Fair Value [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Long-term debt $ 5,077,000,000 $ 5,079,000,000