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Fair Value Measurements - Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Unrealized loss on investment in equity security $ (168) $ (149)  
Market value | Level 2      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Short-term and long-term debt 27,500 27,500 $ 35,500
Carrying value | Level 2      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Short-term and long-term debt $ 27,300 $ 27,300 $ 33,500