BENEFITS PAYABLE - Activity in Benefits Payable (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	$ 4,862	$ 4,668
Less: Reinsurance recoverables	(95)	(70)
Balances, beginning of period, net	4,767	4,598
Incurred related to:
Current year	40,499	34,915
Prior years	(331)	(467)
Total incurred	40,168	34,448
Paid related to:
Current year	(34,625)	(30,204)
Prior years	(4,158)	(3,920)
Total paid	(38,783)	(34,124)
Reinsurance recoverable	68	98
Balances, end of period	6,220	5,020
Retail
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	4,338	3,963
Less: Reinsurance recoverables	(95)	(70)
Balances, beginning of period, net	4,243	3,893
Incurred related to:
Current year	36,762	31,209
Prior years	(366)	(367)
Total incurred	36,396	30,842
Paid related to:
Current year	(31,476)	(27,062)
Prior years	(3,634)	(3,334)
Total paid	(35,110)	(30,396)
Reinsurance recoverable	68	98
Balances, end of period	5,597	4,437
Total IBNR	3,500
Group and Specialty
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	517	568
Incurred related to:
Current year	4,142	4,018
Prior years	35	(41)
Total incurred	4,177	3,977
Paid related to:
Current year	(3,547)	(3,462)
Prior years	(524)	(509)
Total paid	(4,071)	(3,971)
Balances, end of period	623	574
Total IBNR	$ 530
Individual Commercial
Incurred related to:
Total incurred		$ 58