BENEFITS PAYABLE - Favorable Medical Claims Reserve Development (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable medical claims reserve development	$ (336)	$ (503)	$ (483)
Retail
Liability for Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable medical claims reserve development	(386)	(398)	(386)
Group and Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable medical claims reserve development	50	(46)	(40)
Operating Segments | Retail
Liability for Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable medical claims reserve development	(386)	(398)	(386)
Operating Segments | Group and Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable medical claims reserve development	50	(46)	(40)
Operating Segments | Individual Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable medical claims reserve development	0	(57)	(56)
Other Businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable medical claims reserve development	$ 0	$ (2)	$ (1)