BENEFITS PAYABLE - Benefits Expense (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefits payable, net of reinsurance	$ 4,767
Reinsurance recoverable on unpaid claims	95	$ 70	$ 76	$ 85
Total benefits payable, gross	4,862	4,668	4,563	4,976
Retail
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Incurred Claims, Net of Reinsurance	116,475
Cumulative Paid Claims, Net of Reinsurance	112,232
Reinsurance recoverable on unpaid claims	95	70	76	85
Total benefits payable, gross	4,338	3,963	3,506	3,600
Group and Specialty
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Incurred Claims, Net of Reinsurance	16,059
Cumulative Paid Claims, Net of Reinsurance	15,542
Reinsurance recoverable on unpaid claims		0	0	0
Total benefits payable, gross	517	568	579	616
Individual Commercial
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Incurred Claims, Net of Reinsurance	4,236
Cumulative Paid Claims, Net of Reinsurance	4,229
Total benefits payable, gross	7	101	454	$ 741
Operating Segments | Retail
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefits payable, net of reinsurance	4,243
Reinsurance recoverable on unpaid claims	95
Operating Segments | Group and Specialty
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefits payable, net of reinsurance	517
Operating Segments | Individual Commercial
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefits payable, net of reinsurance	7
2016 Claims Incurred Year
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cumulative Paid Claims, Net of Reinsurance			33,784
2016 Claims Incurred Year | Retail
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Incurred Claims, Net of Reinsurance	36,811	36,891	37,212
Cumulative Paid Claims, Net of Reinsurance	36,811	36,841
2016 Claims Incurred Year | Group and Specialty
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Incurred Claims, Net of Reinsurance	5,235	5,234	5,271
Cumulative Paid Claims, Net of Reinsurance	5,234	5,226	4,700
2016 Claims Incurred Year | Individual Commercial
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Incurred Claims, Net of Reinsurance	3,609	3,621	3,677
Cumulative Paid Claims, Net of Reinsurance	3,609	3,606	$ 3,233
2017 Claims Incurred Year | Retail
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Incurred Claims, Net of Reinsurance	38,341	38,604
Cumulative Paid Claims, Net of Reinsurance	38,232	34,781
2017 Claims Incurred Year | Group and Specialty
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Incurred Claims, Net of Reinsurance	5,358	5,403
Cumulative Paid Claims, Net of Reinsurance	5,351	4,843
2017 Claims Incurred Year | Individual Commercial
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Incurred Claims, Net of Reinsurance	627	669
Cumulative Paid Claims, Net of Reinsurance	620	$ 583
2018 Claims Incurred Year | Retail
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Incurred Claims, Net of Reinsurance	41,323
Cumulative Paid Claims, Net of Reinsurance	37,189
2018 Claims Incurred Year | Group and Specialty
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Incurred Claims, Net of Reinsurance	5,466
Cumulative Paid Claims, Net of Reinsurance	4,957
2018 Claims Incurred Year | Individual Commercial
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Incurred Claims, Net of Reinsurance	0
Cumulative Paid Claims, Net of Reinsurance	$ 0