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SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION - Allowance for Credit Losses (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Accounts Receivable, Allowance for Credit Loss [Roll Forward]    
Allowance for credit losses as of beginning of period $ 5 $ 0
Increase in allowance (1) (1)
Allowance for credit losses as of end of period $ 6 $ 1