BENEFITS PAYABLE - Activity in Benefits Payable (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	$ 6,004	$ 4,862
Less: Reinsurance recoverables	(68)	(95)
Balances, beginning of period, net	5,936	4,767
Incurred related to:
Current year	15,913	13,760
Prior years	(284)	(267)
Total incurred	15,629	13,493
Paid related to:
Current year	(10,205)	(8,725)
Prior years	(4,280)	(3,595)
Total paid	(14,485)	(12,320)
Reinsurance recoverable	10	84
Balances, end of period	7,090	6,024
Retail
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	5,363	4,338
Less: Reinsurance recoverables	(68)	(95)
Balances, beginning of period, net	5,295	4,243
Incurred related to:
Current year	14,698	12,606
Prior years	(238)	(283)
Total incurred	14,460	12,323
Paid related to:
Current year	(9,490)	(8,032)
Prior years	(3,778)	(3,133)
Total paid	(13,268)	(11,165)
Reinsurance recoverable	10	84
Balances, end of period	6,497	5,485
Total IBNR included in benefits payable	4,400
Group and Specialty
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	641	517
Incurred related to:
Current year	1,357	1,271
Prior years	(46)	16
Total incurred	1,311	1,287
Paid related to:
Current year	(857)	(803)
Prior years	(502)	(462)
Total paid	(1,359)	(1,265)
Balances, end of period	593	$ 539
Total IBNR included in benefits payable	$ 528