HEALTH CARE REFORM (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended			12 Months Ended
	Sep. 30, 2016	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Premium deficiency reserve		$ 0				$ 176
Premium deficiency reserve, change in estimate				$ 13
Write off of risk corridor receivables			$ 583
Other current assets		3,849	3,438		$ 3,438
Trade accounts payable and accrued expenses		(4,675)	(2,467)		(2,467)
Other long-term assets		2,167	2,226		2,226
Other long-term liabilities		(367)	(263)		(263)
Risk Adjustment Settlement
Supplementary Insurance Information, by Segment [Line Items]
Total net (liability) asset		187	196		196
Reinsurance Recoverables
Supplementary Insurance Information, by Segment [Line Items]
Total net (liability) asset		207	260		260
Health Care Reform
Supplementary Insurance Information, by Segment [Line Items]
Payment of annual health insurance industry fee	$ 916
Amortization of deferred charges				227
2014 Coverage Year
Supplementary Insurance Information, by Segment [Line Items]
Increase (decrease) in premiums receivable					(37)
2015 Coverage Year
Supplementary Insurance Information, by Segment [Line Items]
Proceeds from reinsurance recoverable		60		213
Proceeds from risk adjustment settlements		2		$ 6
2014 and 2015 ACA Products
Supplementary Insurance Information, by Segment [Line Items]
Write off of risk corridor receivables			415
2016 Coverage Year | Risk Adjustment Settlement
Supplementary Insurance Information, by Segment [Line Items]
Premiums receivable		320	307		307
Other current assets		0	0		0
Trade accounts payable and accrued expenses		(124)	(117)		(117)
Net current asset		196	190		190
Other long-term assets		0	6		6
Total net (liability) asset		196	196		196
2016 Coverage Year | Reinsurance Recoverables
Supplementary Insurance Information, by Segment [Line Items]
Premiums receivable		0	0		0
Other current assets		207	260		260
Trade accounts payable and accrued expenses		0	0		0
Net current asset		207	260		260
Other long-term assets		0	0		0
Total net (liability) asset		207	260		260
Current Coverage Year | Risk Adjustment Settlement
Supplementary Insurance Information, by Segment [Line Items]
Other long-term assets		17	0		0
Other long-term liabilities		(26)	0		0
Total net (liability) asset		(9)	0		0
Current Coverage Year | Reinsurance Recoverables
Supplementary Insurance Information, by Segment [Line Items]
Other long-term assets		0	0		0
Other long-term liabilities		0	0		0
Total net (liability) asset		$ 0	$ 0		$ 0