BENEFITS PAYABLE - Favorable Medical Claims Reserve Development (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable medical claims reserve development	$ (313)	$ (336)	$ (503)
Retail
Liability for Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable medical claims reserve development	(266)	(386)	(398)
Group and Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable medical claims reserve development	(47)	50	(46)
Operating Segments | Retail
Liability for Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable medical claims reserve development	(266)	(386)	(398)
Operating Segments | Group and Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable medical claims reserve development	(47)	50	(46)
Operating Segments | Individual Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable medical claims reserve development	0	0	(57)
Other Businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable medical claims reserve development	$ 0	$ 0	$ (2)