SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Allowance for Credit Loss Roll-forward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit loss at beginning of year	$ 88	$ 92	$ 84
Accounts Receivable, Credit Loss Expense (Reversal)	7	3	9
Accounts Receivable, Allowance For Credit Loss, Write-offs, Net Of Recoveries	0	(7)	(1)
Allowance for credit loss at end of year	$ 95	$ 88	$ 92