SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Allowance for Doubtful Accounts Roll-forward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for doubtful accounts at beginning of year	$ 92	$ 84	$ 93
Additional provisions for the year	3	9	4
Uncollectible accounts written off net of recoveries	(7)	(1)	(13)
Allowance for doubtful accounts at end of year	$ 88	$ 92	$ 84