SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Allowance for Doubtful Accounts Roll-forward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Allowance for doubtful accounts at beginning of year	$ 93	$ 103	$ 98
Additional provisions for the year	4	14	40
Uncollectible accounts written off net of recoveries	(13)	(24)	(35)
Allowance for doubtful accounts at end of year	$ 84	$ 93	$ 103