BENEFITS PAYABLE - Narrative (Details) claim in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) claim	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Claims Development [Line Items]
Favorable medical claims reserve development related to prior fiscal years | $	$ 313	$ 336	$ 503
Retail
Claims Development [Line Items]
Total IBNR | $	4,700
Group and Specialty
Claims Development [Line Items]
Total IBNR | $	$ 594
2020 Claims Incurred Year | Retail
Claims Development [Line Items]
Cumulative number of reported claims	133.0
2020 Claims Incurred Year | Group and Specialty
Claims Development [Line Items]
Cumulative number of reported claims	8.6
2019 Claims Incurred Year | Retail
Claims Development [Line Items]
Cumulative number of reported claims	128.8
2019 Claims Incurred Year | Group and Specialty
Claims Development [Line Items]
Cumulative number of reported claims	10.0
2018 Claims Incurred Year | Retail
Claims Development [Line Items]
Cumulative number of reported claims	109.9
2018 Claims Incurred Year | Group and Specialty
Claims Development [Line Items]
Cumulative number of reported claims	10.9