BENEFITS PAYABLE - Activity in Benefits Payable (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	$ 6,004	$ 4,862	$ 4,668
Less: Reinsurance recoverables	(68)	(95)	(70)
Balances, beginning of period, net	5,936	4,767	4,598
Incurred related to:
Current year	61,941	54,193	46,385
Prior years	(313)	(336)	(503)
Total incurred	61,628	53,857	45,882
Paid related to:
Current year	(54,003)	(48,421)	(41,736)
Prior years	(5,418)	(4,267)	(3,977)
Total paid	(59,421)	(52,688)	(45,713)
Reinsurance recoverable	0	68	95
Balances, end of period	8,143	6,004	4,862
Retail
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	5,363	4,338	3,963
Less: Reinsurance recoverables	(68)	(95)	(70)
Balances, beginning of period, net	5,295	4,243	3,893
Incurred related to:
Current year	56,821	48,983	41,323
Prior years	(266)	(386)	(398)
Total incurred	56,555	48,597	40,925
Paid related to:
Current year	(49,586)	(43,831)	(37,189)
Prior years	(4,836)	(3,714)	(3,386)
Total paid	(54,422)	(47,545)	(40,575)
Reinsurance recoverable	0	68	95
Balances, end of period	7,428	5,363	4,338
Group and Specialty
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	641	517	568
Incurred related to:
Current year	5,576	5,708	5,466
Prior years	(47)	50	(46)
Total incurred	5,529	5,758	5,420
Paid related to:
Current year	(4,873)	(5,081)	(4,957)
Prior years	(582)	(553)	(514)
Total paid	(5,455)	(5,634)	(5,471)
Balances, end of period	$ 715	$ 641	$ 517