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Fair Value Measurement - Summary of Financial Instrument Measured on a Basis Other Than Fair Value (Detail) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total [1] $ 26,354 $ 25,700
Senior Notes [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total [1] 23,622 23,560
Senior Notes and Convertible Senior Debentures Included Under Short-Term Liabilities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total [1] $ 2,732 $ 2,140
[1] The fair value was estimated based on quoted market prices, where available.