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Fair Value Measurements - Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Unrealized loss on investment in equity security $ 64 $ 19  
Cash 2,000 2,000 $ 2,400
Cash equivalents 4,100 4,100 481
Market value | Level 2      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Short-term and long-term debt 29,300 29,300 35,500
Carrying value | Level 2      
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]      
Short-term and long-term debt $ 29,100 $ 29,100 $ 33,500