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Fair Value Measurements and Financial Instruments - Carrying Value and Estimated Fair Value of Long-term Debt (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Long-term Debt    
Debt Instrument [Line Items]    
Carrying amount of hedged item $ 404 $ 888
Cumulative hedging adjustment included in the carrying amount 6 (10)
Carrying Value    
Debt Instrument [Line Items]    
Carrying value of long-term debt 7,648 6,354
Fair Value, Inputs, Level 2    
Debt Instrument [Line Items]    
Estimated fair value of long-term debt $ 8,159 $ 6,434