HEALTH CARE REFORM (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Nov. 02, 2017
Supplementary Insurance Information, by Segment [Line Items]
Litigation recoveries sought					$ 611
Other current assets		$ 4,059		$ 2,949
Trade accounts payable and accrued expenses		(6,266)		(4,069)
Other long-term assets		1,805		2,166
Other long-term liabilities		(303)		(237)
Risk Adjustment Settlement
Supplementary Insurance Information, by Segment [Line Items]
Total net (liability) asset		(24)		(13)
Reinsurance Recoverables
Supplementary Insurance Information, by Segment [Line Items]
Total net (liability) asset		0		44
Health Care Reform
Supplementary Insurance Information, by Segment [Line Items]
Amortization of deferred charges		263
Prior Coverage Years
Supplementary Insurance Information, by Segment [Line Items]
Proceeds from risk corridor settlements		46	$ 62
Prior Coverage Years | Risk Adjustment Settlement
Supplementary Insurance Information, by Segment [Line Items]
Premiums receivable		65		62
Other current assets		0		0
Trade accounts payable and accrued expenses		(79)		(80)
Net current (liability) asset		(14)		(18)
Other long-term assets		0		5
Total net (liability) asset		(14)		(13)
Prior Coverage Years | Reinsurance Recoverables
Supplementary Insurance Information, by Segment [Line Items]
Premiums receivable		0		0
Other current assets		0		44
Trade accounts payable and accrued expenses		0		0
Net current (liability) asset		0		44
Other long-term assets		0		0
Total net (liability) asset		0		44
Current Coverage Year | Risk Adjustment Settlement
Supplementary Insurance Information, by Segment [Line Items]
Other long-term assets		1		0
Other long-term liabilities		(11)		0
Net long-term liability		(10)		0
Total net (liability) asset		(10)		0
Current Coverage Year | Reinsurance Recoverables
Supplementary Insurance Information, by Segment [Line Items]
Other long-term assets		0		0
Other long-term liabilities		0		0
Net long-term liability		0		0
Total net (liability) asset		$ 0		$ 0
Forecast | Health Care Reform
Supplementary Insurance Information, by Segment [Line Items]
Payment of annual health insurance industry fee	$ 1,050