BENEFITS PAYABLE - Incurred and Paid Claims Development (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Claims Development [Line Items]
Benefits payable, net of reinsurance	$ 8,143
Retail
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	146,587
Cumulative Paid Claims, Net of Reinsurance	139,159
Benefits payable, net of reinsurance	7,428
Group and Specialty
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	16,738
Cumulative Paid Claims, Net of Reinsurance	16,023
Benefits payable, net of reinsurance	715
2018 Claims Incurred Year | Retail
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	40,946	$ 40,984	$ 41,323
Cumulative Paid Claims, Net of Reinsurance	40,946	40,841	37,189
2018 Claims Incurred Year | Group and Specialty
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	5,505	5,501	5,466
Cumulative Paid Claims, Net of Reinsurance	5,505	5,487	$ 4,957
2019 Claims Incurred Year | Retail
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	48,820	48,983
Cumulative Paid Claims, Net of Reinsurance	48,627	43,831
2019 Claims Incurred Year | Group and Specialty
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	5,657	5,708
Cumulative Paid Claims, Net of Reinsurance	5,645	$ 5,081
2020 Claims Incurred Year | Retail
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	56,821
Cumulative Paid Claims, Net of Reinsurance	49,586
2020 Claims Incurred Year | Group and Specialty
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	5,576
Cumulative Paid Claims, Net of Reinsurance	$ 4,873