BENEFITS PAYABLE - Activity in Benefits Payable (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	$ 8,143	$ 6,004
Less: Reinsurance recoverables	0	(68)
Balances, beginning of period, net	8,143	5,936
Acquisitions	42	0
Incurred related to:
Current year	17,851	15,913
Prior years	(555)	(284)
Total incurred	17,296	15,629
Paid related to:
Current year	(10,842)	(10,205)
Prior years	(5,988)	(4,280)
Total paid	(16,830)	(14,485)
Reinsurance recoverable	0	10
Balances, end of period	8,651	7,090
Retail
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	7,428	5,363
Less: Reinsurance recoverables	0	(68)
Balances, beginning of period, net	7,428	5,295
Acquisitions	42	0
Incurred related to:
Current year	16,762	14,698
Prior years	(463)	(238)
Total incurred	16,299	14,460
Paid related to:
Current year	(10,330)	(9,490)
Prior years	(5,401)	(3,778)
Total paid	(15,731)	(13,268)
Reinsurance recoverable	0	10
Balances, end of period	8,038	6,497
Total IBNR included in benefits payable	5,000
Group and Specialty
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	715	641
Incurred related to:
Current year	1,237	1,357
Prior years	(92)	(46)
Total incurred	1,145	1,311
Paid related to:
Current year	(660)	(857)
Prior years	(587)	(502)
Total paid	(1,247)	(1,359)
Balances, end of period	613	$ 593
Total IBNR included in benefits payable	$ 532