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Fair Value Measurements - Reported (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Reported Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt $ 11,934 $ 6,966
SMR liability 90 91
Estimate of Fair Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt 12,442 7,718
SMR liability $ 100 $ 104