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Fair Value Measurements - Financial Instruments (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Carrying Value    
Financial Instruments    
Long-term debt, excluding current portion - Fair Value $ 12,211 $ 12,096
Fair Value    
Financial Instruments    
Long-term debt, excluding current portion - Fair Value $ 12,659 $ 12,535