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Other Financial Information - Additional Information (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale debt securities $ 22,561 $ 29,587
Prepaid and other current assets    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale debt securities $ 520