SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) - USD ($)		12 Months Ended
	Feb. 16, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Significant Accounting Policies [Line Items]
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%
Contract liabilities		$ 0
Deferred costs		0
Pharmaceutical rebates		$ 1,300,000,000	$ 1,300,000,000
Typical term (in years) of short-duration employer-group prepaid health services policies		1 year
Lease liabilities		$ 448,000,000
Right-of-use assets		410,000,000
Aetna
Significant Accounting Policies [Line Items]
Merger breakup fee received	$ 1,000,000,000
Tricare South Region Contract
Significant Accounting Policies [Line Items]
TRICARE health care cost reimbursements		6,400,000,000	5,600,000,000	$ 3,400,000,000
TRICARE health care cost payments		6,500,000,000
Difference between TRICARE health care costs payments and reimbursements		$ 63,000,000	(38,000,000)	(11,000,000)
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			1,040,000,000.00
CMS reinsurance subsidies and discounts
Significant Accounting Policies [Line Items]
Subsidy and discount payments		$ 11,800,000,000	10,300,000,000	10,200,000,000
Subsidy reimbursements		11,200,000,000	9,600,000,000	12,100,000,000
Subsidy and discount payments in excess of reimbursements		$ 600,000,000	700,000,000	$ 1,900,000,000
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
Services
Significant Accounting Policies [Line Items]
Accounts receivable, net, current		$ 141,000,000	$ 123,000,000
Bad-debt expense		0
Contract with customer, asset, net		$ 0
ASU 2016-02
Significant Accounting Policies [Line Items]
Lease liabilities					$ 470,000,000
Right-of-use assets					$ 436,000,000