BENEFITS PAYABLE - Incurred and Paid Claims Development (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Claims Development [Line Items]
Benefits payable, net of reinsurance	$ 5,936
Retail
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	128,277
Cumulative Paid Claims, Net of Reinsurance	122,982
Benefits payable, net of reinsurance	5,295
Group and Specialty
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	16,581
Cumulative Paid Claims, Net of Reinsurance	15,940
Benefits payable, net of reinsurance	641
2017 Claims Incurred Year | Retail
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	38,310	$ 38,341	$ 38,604
Cumulative Paid Claims, Net of Reinsurance	38,310	38,232	34,781
2017 Claims Incurred Year | Group and Specialty
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	5,372	5,358	5,403
Cumulative Paid Claims, Net of Reinsurance	5,372	5,351	$ 4,843
2018 Claims Incurred Year | Retail
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	40,984	41,323
Cumulative Paid Claims, Net of Reinsurance	40,841	37,189
2018 Claims Incurred Year | Group and Specialty
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	5,501	5,466
Cumulative Paid Claims, Net of Reinsurance	5,487	$ 4,957
2019 Claims Incurred Year | Retail
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	48,983
Cumulative Paid Claims, Net of Reinsurance	43,831
2019 Claims Incurred Year | Group and Specialty
Claims Development [Line Items]
Incurred Claims, Net of Reinsurance	5,708
Cumulative Paid Claims, Net of Reinsurance	$ 5,081