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Marketable Securities - Changes in Value of Marketable Securities Reported in Current Assets on Consolidated Balance Sheets (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Beginning Balance $ 335 $ 335
Purchase 99  
Sale 0 0
Change in fair value included in OCI (3)  
Change in fair value recognized in earnings 0 0
Ending Balance 431 335
U.S. Treasury Bonds/Bills [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Beginning Balance 335 335
Purchase 99  
Sale 0 0
Change in fair value included in OCI (3)  
Change in fair value recognized in earnings 0 0
Ending Balance $ 431 $ 335