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Fixed Assets - Depreciation Expense Relating to Fixed Assets (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Property Plant And Equipment [Abstract]        
Depreciation expense relating to fixed assets $ 499 $ 516 $ 1,525 $ 1,564