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Marketable Securities - Additional Information (Details) - United States Government Agencies Corporate Debt Securities And Certificates Of Deposit [Member]
$ in Millions
Dec. 31, 2017
USD ($)
Schedule of Available-for-sale Securities [Line Items]  
Marketable securities, maturing in one year or less $ 27
Marketable securities, maturing after one year through five years 5
Marketable securities, maturing after five years through ten years $ 3