BENEFITS PAYABLE - Activity in Benefits Payable (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning balance	$ 4,668	$ 4,563
Less: Reinsurance recoverables	(70)	(76)
Balances, beginning of period, net			$ 4,598	$ 4,487
Incurred related to:
Current year	11,947	11,580
Prior years	(267)	(231)
Total incurred	11,680	11,349
Paid related to:
Current year	(7,775)	(7,695)
Prior years	(3,619)	(3,451)
Total paid	(11,394)	(11,146)
Reinsurance recoverable	77	71
Ending balance	4,961	4,761
Retail
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning balance	3,963	3,507
Less: Reinsurance recoverables	(70)	(76)
Balances, beginning of period, net			$ 3,893	$ 3,431
Incurred related to:
Current year	10,739	10,255
Prior years	(187)	(204)
Total incurred	10,552	10,051
Paid related to:
Current year	(7,119)	(7,014)
Prior years	(3,082)	(2,572)
Total paid	(10,201)	(9,586)
Reinsurance recoverable	77	71
Ending balance	4,321	3,967
Total IBNR	2,800
Group and Specialty
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning balance	568	578
Incurred related to:
Current year	1,307	1,306
Prior years	(34)	(20)
Total incurred	1,273	1,286
Paid related to:
Current year	(802)	(824)
Prior years	(463)	(493)
Total paid	(1,265)	(1,317)
Ending balance	576	547
Total IBNR	502
Individual Commercial
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning balance	101	454
Incurred related to:
Current year	0	195
Prior years	(44)	(6)
Total incurred	(44)	189
Paid related to:
Current year	0	(59)
Prior years	(29)	(363)
Total paid	(29)	(422)
Ending balance	28	$ 221
Total IBNR	$ 19