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Fair Value Measurements (Narrative) (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Fair Value Disclosures [Abstract]    
Net Assets Measured At Fair Value On A Recurring Basis $ 2,200 $ 2,600
Net Assets Measured At Fair Value On A Recurring Basis Measured Using Unobservable Input And Classified As Level3 $ 1 $ 7