SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Details (Detail) - USD ($)		12 Months Ended
	Feb. 16, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019
Significant Accounting Policies [Line Items]
Transaction costs (break up fee received)		$ 0	$ (936,000,000)	$ 104,000,000
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		$ 5,600,000,000	3,400,000,000	3,300,000,000
Bad-debt expense		36,000,000	20,000,000	39,000,000
Contract liabilities		0
Deferred costs		0
Pharmaceutical rebates		$ 1,300,000,000	1,200,000,000
Typical term (in years) of short-duration employer-group prepaid health services policies		1 year
Goodwill		$ 3,897,000,000	3,281,000,000	3,272,000,000
Premium deficiency reserve				176,000,000
Health care reform, premium deficiency reserve, change in estimate				208,000,000
Health care reform, premium deficiency reserve, current period loss				384,000,000
Tricare South Region Contract
Significant Accounting Policies [Line Items]
TRICARE Health care cost reimbursements		5,600,000,000	3,400,000,000	3,300,000,000
Difference between TRICARE health care costs payments and reimbursements		$ 38,000,000	11,000,000	25,000,000
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
Health Care Reform
Significant Accounting Policies [Line Items]
Payment of annual health insurance industry fee		$ 1,040,000,000		916,000,000
CMS Reinsurance Subsidies Or Discounts
Significant Accounting Policies [Line Items]
Subsidy and discount reimbursements		9,600,000,000	12,100,000,000	11,100,000,000
Subsidy and discount payments		10,300,000,000	10,200,000,000	10,000,000,000
Subsidy and discount payments in excess of reimbursements		$ 700,000,000	1,900,000,000	$ 1,100,000,000
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
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
Service
Significant Accounting Policies [Line Items]
Accounts receivable, net, current		$ 123,000,000	$ 180,000,000
Bad-debt expense		0
Contract with customer, asset, net		0
Subsequent Event | Accounting Standards Update 2016-02
Significant Accounting Policies [Line Items]
Operating lease, liability					$ 470,000,000
Operating lease, right-of-use asset					$ 470,000,000
Clinical
Significant Accounting Policies [Line Items]
Goodwill		524,000,000
Provider
Significant Accounting Policies [Line Items]
Goodwill		$ 730,000,000