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Schedule II Valuation and Qualifying Accounts (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at beginning of period $ 6,246 $ 6,886  
Deductions (15,707) (17,605)  
Balance at end of period 4,904 6,246 $ 6,886
Allowance For Doubtful Accounts [Member]      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at beginning of period 209 232 232
Charged to costs and expenses (11) (16) 13
Charged to other accounts 2   (9)
Deductions   (7) (4)
Balance at end of period 200 209 232
Valuation Allowance in Tax Carryforward Losses And Deductions [Member]      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at beginning of period 1,974 1,633 1,504
Charged to costs and expenses 670 555 407
Charged to other accounts     5
Deductions (97) (214) (283)
Balance at end of period $ 2,547 $ 1,974 $ 1,633