INCOME TAXES - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Unrecognized tax benefits
Unrecognized tax benefit — January 1	$ 56	$ 45	$ 32
Gross increases — tax positions in current period	2	3	1
Gross increases — tax positions in prior periods	48	11	19
Gross decreases — tax positions in prior periods	(11)	(3)	(3)
Reduction due to lapse in statute of limitations	0	0	(2)
Reduction due to settlements during the current period	(11)	0	(2)
Unrecognized tax benefit — December 31	$ 84	$ 56	$ 45