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Fair Value Measurements - Additional Information (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Fair Value Disclosures [Abstract]    
Estimated fair values of debt instruments $ 15,400 $ 16,800
Outstanding principal amount of debt instruments 15,312 15,498
Unamortized discounts and issuance costs $ 1,208 $ 1,235