BENEFITS PAYABLE - Activity in Benefits Payable (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 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	13,760	11,947
Prior years	(267)	(267)
Total incurred	13,493	11,680
Paid related to:
Current year	(8,725)	(7,775)
Prior years	(3,595)	(3,619)
Total paid	(12,320)	(11,394)
Reinsurance recoverable	84	77
Balances, end of period	6,024	4,961
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	12,606	10,739
Prior years	(283)	(187)
Total incurred	12,323	10,552
Paid related to:
Current year	(8,032)	(7,119)
Prior years	(3,133)	(3,082)
Total paid	(11,165)	(10,201)
Reinsurance recoverable	84	77
Balances, end of period	5,485	4,321
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	1,271	1,307
Prior years	16	(34)
Total incurred	1,287	1,273
Paid related to:
Current year	(803)	(802)
Prior years	(462)	(463)
Total paid	(1,265)	(1,265)
Balances, end of period	539	576
Total IBNR	$ 449
Individual Commercial
Incurred related to:
Total incurred		$ 44