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