BENEFITS PAYABLE - Activity in Benefits Payable (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	$ 4,668	$ 4,563
Less: Reinsurance recoverables	(98)	(70)	$ (70)	$ (76)
Balances, beginning of period, net	4,598	4,487
Incurred related to:
Current year	34,915	33,318
Prior years	(467)	(430)
Total incurred	34,448	32,888
Paid related to:
Current year	(30,204)	(28,741)
Prior years	(3,920)	(3,745)
Total paid	(34,124)	(32,486)
Reinsurance recoverable	98	70	70	76
Balances, end of period	5,020	4,959
Retail
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	3,963	3,507
Less: Reinsurance recoverables	(98)	(70)	(70)	(76)
Balances, beginning of period, net	3,893	3,431
Incurred related to:
Current year	31,209	29,356
Prior years	(367)	(339)
Total incurred	30,842	29,017
Paid related to:
Current year	(27,062)	(25,460)
Prior years	(3,334)	(2,822)
Total paid	(30,396)	(28,282)
Reinsurance recoverable	98	70	$ 70	$ 76
Balances, end of period	4,437	4,236
Total IBNR	2,800
Group and Specialty
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	568	578
Incurred related to:
Current year	4,018	3,996
Prior years	(41)	(44)
Total incurred	3,977	3,952
Paid related to:
Current year	(3,462)	(3,452)
Prior years	(509)	(517)
Total paid	(3,971)	(3,969)
Balances, end of period	574	561
Total IBNR	490
Individual Commercial
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balances, beginning of period	101	454
Incurred related to:
Current year	0	502
Prior years	(58)	(46)
Total incurred	(58)	456
Paid related to:
Current year	0	(393)
Prior years	(34)	(383)
Total paid	(34)	(776)
Balances, end of period	9	$ 134
Total IBNR	$ 2