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SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Allowance for doubtful accounts, deducted from accounts receivable      
Income tax valuation allowance, deducted from deferred tax assets      
Balance at Beginning of Period $ 43 $ 32 $ 28
Charged to Costs and Expenses 108 95 83
Charged to Other Accounts 0 0 0
Deductions (84) (84) (79)
Balance at End of Period 67 43 32
Income tax valuation allowance, deducted from deferred tax assets      
Income tax valuation allowance, deducted from deferred tax assets      
Balance at Beginning of Period 242 3,794 1,863
Charged to Costs and Expenses 24 (3,543) 1,934
Charged to Other Accounts 0 (9) (128)
Deductions 0 0 125
Balance at End of Period $ 266 $ 242 $ 3,794