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)