Segment Information - Revenues by Product Type (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue from External Customer [Line Items]
Premiums	$ 30,626	$ 29,642	$ 27,214
Mail order pharmacy revenues	2,966	2,536	2,239
Other	378	374	373
Revenues from external customers	38,352	36,666	33,594
Medical [Member]
Revenue from External Customer [Line Items]
Premiums	23,295	22,696	20,709
Fees	4,368	4,107	3,767
Total premiums and fees	27,663	26,803	24,476
Guaranteed Cost [Member]
Revenue from External Customer [Line Items]
Premiums	4,610	4,761	4,600
Experience Rated [Member]
Revenue from External Customer [Line Items]
Premiums	2,383	2,329	2,322
Stop Loss [Member]
Revenue from External Customer [Line Items]
Premiums	3,082	2,701	2,318
International Health Care [Member]
Revenue from External Customer [Line Items]
Premiums	1,859	1,834	1,827
Dental [Member]
Revenue from External Customer [Line Items]
Premiums	1,586	1,392	1,257
Medicare [Member]
Revenue from External Customer [Line Items]
Premiums	6,621	6,142	5,660
Medicaid [Member]
Revenue from External Customer [Line Items]
Premiums	1,146	1,102	515
Medicare Part D [Member]
Revenue from External Customer [Line Items]
Premiums	1,122	1,589	1,405
Other Medical [Member]
Revenue from External Customer [Line Items]
Premiums	886	846	805
Disability [Member]
Revenue from External Customer [Line Items]
Total premiums and fees	2,045	1,899	1,767
Life, Accident and Supplemental Health [Member]
Revenue from External Customer [Line Items]
Total premiums and fees	$ 5,300	$ 5,054	$ 4,739

Revenue excluding net investment income and realized investment results.

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned, plus amount of fees and commissions from financial services and banking activities and correspondent clearing. Includes fees from depositor accounts, credit cards, merchant discounts, and fiduciary and trust activities, guarantee fees, investment advisory, management, and administrative fees, transfer agent fees, and insurance commissions.

+ References

No definition available.

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Reflects premiums and fees related to products within the Other Operations segment, plus other income for all products. Other income reflects the sum of all other revenue and income recognized by the entity in the period not otherwise specified in the income statement. Products within the Other Operations segment include corporate-owned life insurance, run-off retirement and run-off reinsurance products.

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No definition available.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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Amount of fees and commissions from financial services and banking activities and correspondent clearing. Includes fees from depositor accounts, credit cards, merchant discounts, and fiduciary and trust activities, guarantee fees, investment advisory, management, and administrative fees, transfer agent fees, and insurance commissions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
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Aggregate revenue during the period from the sale of goods in the normal course of business, after deducting returns, allowances and discounts.

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