BENEFITS PAYABLE - Activity in Benefits Payable (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 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	52,529	45,693
Prior years	(768)	(278)
Total incurred	51,761	45,415
Paid related to:
Current year	(44,370)	(37,810)
Prior years	(6,818)	(5,334)
Total paid	(51,188)	(43,144)
Reinsurance recoverable	0	1
Balances, end of period	8,758	8,208
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	49,247	42,186
Prior years	(673)	(235)
Total incurred	48,574	41,951
Paid related to:
Current year	(41,721)	(34,946)
Prior years	(6,216)	(4,759)
Total paid	(47,937)	(39,705)
Reinsurance recoverable	0	1
Balances, end of period	8,107	7,542
Total IBNR included in benefits payable	5,400
Group and Specialty
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	715	641
Incurred related to:
Current year	3,769	3,929
Prior years	(95)	(43)
Total incurred	3,674	3,886
Paid related to:
Current year	(3,136)	(3,286)
Prior years	(602)	(575)
Total paid	(3,738)	(3,861)
Balances, end of period	651	$ 666
Total IBNR included in benefits payable	$ 564