SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Additional Details (Detail) - USD ($)		3 Months Ended			12 Months Ended
	Feb. 16, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant Accounting Policies [Line Items]
Transaction costs (break up fee received)					$ (936,000,000)	$ 104,000,000	$ 23,000,000
Premiums reduction, write off of risk corridor receivables		$ 583,000,000
Member co-share amounts and government subsidies					$ 13,500,000,000	13,400,000,000	12,300,000,000
Number of days notice for employer group to cancel short-duration prepaid health services policies					30 days
Consumer discounts on brand name prescription drugs					50.00%
TRICARE Health care cost payments					$ 3,400,000,000	3,300,000,000	3,300,000,000
Pharmaceutical rebates		889,000,000			$ 1,200,000,000	889,000,000
Typical term (in years) of short-duration employer-group prepaid health services policies					1 year
Premium deficiency reserve							176,000,000
Health care reform, premium deficiency reserve, change in estimate			$ 208,000,000
Outstanding borrowings, line of credit					$ 0
Investment income					405,000,000	389,000,000	474,000,000
Benefits expense					43,496,000,000	45,007,000,000	44,269,000,000
Net cash provided by operating activities					4,051,000,000	1,936,000,000	868,000,000
Net cash used in financing activities					(945,000,000)	732,000,000	(552,000,000)
Tricare South Region Contract
Significant Accounting Policies [Line Items]
TRICARE Health care cost reimbursements					3,400,000,000	3,300,000,000	3,300,000,000
Difference between TRICARE health care costs payments and reimbursements					$ 11,000,000	(25,000,000)	(4,000,000)
Equipment | Minimum
Significant Accounting Policies [Line Items]
Useful life					3 years
Equipment | Maximum
Significant Accounting Policies [Line Items]
Useful life					10 years
Computer Software | Minimum
Significant Accounting Policies [Line Items]
Useful life					3 years
Computer Software | Maximum
Significant Accounting Policies [Line Items]
Useful life					5 years
Building | Minimum
Significant Accounting Policies [Line Items]
Useful life					10 years
Building | Maximum
Significant Accounting Policies [Line Items]
Useful life					20 years
Commercial Medical
Significant Accounting Policies [Line Items]
Percentage of premiums and services revenue					13.00%
CMS Reinsurance Subsidies Or Discounts
Significant Accounting Policies [Line Items]
Subsidy and discount reimbursements					$ 12,100,000,000	11,100,000,000	8,600,000,000
Subsidy and discount payments					10,200,000,000	10,000,000,000	8,900,000,000
Subsidy and discount payments in excess of reimbursements					$ 1,900,000,000	1,100,000,000	(361,000,000)
Settlement period after close					9 months
CMS Brand Name Prescription Drug Discounts | Minimum
Significant Accounting Policies [Line Items]
Settlement period after close					14 months
CMS Brand Name Prescription Drug Discounts | Maximum
Significant Accounting Policies [Line Items]
Settlement period after close					18 months
MCCI
Significant Accounting Policies [Line Items]
Interest rate on receivable					10.00%
Maximum borrowing capacity					$ 55,000,000
Outstanding borrowings, line of credit		0			18,000,000	0
Advances to affiliate		6,000,000			3,000,000	6,000,000
Benefits expense					1,100,000,000	1,100,000,000	1,000,000,000
Health and Human Services | Reinsurance Contribution
Significant Accounting Policies [Line Items]
Government and health plan member payments					76,000,000	373,000,000	409,000,000
Member co-share amounts and government subsidies					32,000,000	345,000,000	478,000,000
Government and health plan member subsidies, amount in excess of payments					(44,000,000)	(28,000,000)	69,000,000
New Accounting Pronouncement, Early Adoption, Effect | Accounting Standards Update 2016-09
Significant Accounting Policies [Line Items]
Net cash provided by operating activities				$ 20,000,000
Net cash used in financing activities				$ (20,000,000)
Operating Segments | Healthcare Services
Significant Accounting Policies [Line Items]
Transaction costs (break up fee received)					0	0	0
Investment income					35,000,000	30,000,000	0
Benefits expense					0	0	$ 0
Other long-term assets | MCCI
Significant Accounting Policies [Line Items]
Related party receivable		$ 314,000,000			$ 349,000,000	$ 314,000,000
Revenue From Premiums and Investment Income | External Revenues
Significant Accounting Policies [Line Items]
Premiums and investment income as a percent of consolidated external revenues					98.00%
Settled Litigation | U.S. Department Of Justice vs. Humana And Aetna
Significant Accounting Policies [Line Items]
Transaction costs (break up fee received)	$ 1,000,000,000