SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) - USD ($)
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12 Months Ended |
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Feb. 16, 2017 |
Dec. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
Jan. 01, 2019 |
| Significant Accounting Policies [Line Items] |
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| Number of days notice for employer group to cancel short-duration prepaid health services policies |
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30 days
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| Consumer discounts on brand name prescription drugs |
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50.00%
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| Contract liabilities |
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$ 0
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| Deferred costs |
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0
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| Pharmaceutical rebates |
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$ 1,300,000,000
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$ 1,300,000,000
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| Typical term (in years) of short-duration employer-group prepaid health services policies |
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1 year
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| Lease liabilities |
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$ 448,000,000
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| Right-of-use assets |
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410,000,000
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| Aetna |
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| Significant Accounting Policies [Line Items] |
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| Merger breakup fee received |
$ 1,000,000,000
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| Tricare South Region Contract |
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| Significant Accounting Policies [Line Items] |
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| TRICARE health care cost reimbursements |
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6,400,000,000
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5,600,000,000
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$ 3,400,000,000
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| TRICARE health care cost payments |
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6,500,000,000
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| Difference between TRICARE health care costs payments and reimbursements |
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$ 63,000,000
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(38,000,000)
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(11,000,000)
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| Equipment | Minimum |
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| Significant Accounting Policies [Line Items] |
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| Useful life |
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3 years
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| Equipment | Maximum |
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| Significant Accounting Policies [Line Items] |
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| Useful life |
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10 years
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| Computer Software | Minimum |
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| Significant Accounting Policies [Line Items] |
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| Useful life |
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3 years
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| Computer Software | Maximum |
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| Significant Accounting Policies [Line Items] |
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| Useful life |
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5 years
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| Building | Minimum |
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| Significant Accounting Policies [Line Items] |
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| Useful life |
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10 years
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| Building | Maximum |
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| Significant Accounting Policies [Line Items] |
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| Useful life |
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20 years
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| Health Care Reform |
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| Significant Accounting Policies [Line Items] |
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| Payment of annual health insurance industry fee |
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1,040,000,000.00
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| CMS reinsurance subsidies and discounts |
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| Significant Accounting Policies [Line Items] |
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| Subsidy and discount payments |
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$ 11,800,000,000
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10,300,000,000
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10,200,000,000
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| Subsidy reimbursements |
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11,200,000,000
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9,600,000,000
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12,100,000,000
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| Subsidy and discount payments in excess of reimbursements |
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$ 600,000,000
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700,000,000
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$ 1,900,000,000
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| Settlement period after close |
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9 months
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| CMS brand name prescription drug discounts | Minimum |
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| Significant Accounting Policies [Line Items] |
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| Settlement period after close |
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14 months
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| CMS brand name prescription drug discounts | Maximum |
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| Significant Accounting Policies [Line Items] |
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| Settlement period after close |
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18 months
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| Services |
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| Significant Accounting Policies [Line Items] |
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| Accounts receivable, net, current |
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$ 141,000,000
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$ 123,000,000
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| Bad-debt expense |
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0
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| Contract with customer, asset, net |
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$ 0
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| ASU 2016-02 |
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| Significant Accounting Policies [Line Items] |
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| Lease liabilities |
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$ 470,000,000
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| Right-of-use assets |
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$ 436,000,000
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