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Fair Value of Financial Instruments - Balance Sheet Grouping (Details) - USD ($)
$ in Millions
Jun. 30, 2019
Dec. 31, 2018
Reported Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt and lease obligation, including current maturities $ 6,183 $ 6,043
Estimate of Fair Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt and lease obligation, including current maturities 6,121 5,943
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt and lease obligation, including current maturities 1,507 1,620
Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt and lease obligation, including current maturities $ 4,614 $ 4,323