Fair Value Measurements - Separate Account Assets Priced at NAV (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	$ 67
Minimum [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period	45 days
Maximum [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period	90 days
Fair Value, Measurements, Recurring [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Redemption Frequency	Quarterly redemption frequency
Separate Account Assets [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	$ 315
Separate Account Assets [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Fair Value	782	$ 774
Security Partnerships [Member] \| Separate Account Assets [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	$ 315
Redemption Frequency	Not applicable
Security Partnerships [Member] \| Separate Account Assets [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 487	$ 458
Real Estate Funds [Member] \| Separate Account Assets [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	$ 0
Redemption Frequency	Quarterly
Real Estate Funds [Member] \| Separate Account Assets [Member] \| Minimum [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period	45 days	45 days
Real Estate Funds [Member] \| Separate Account Assets [Member] \| Maximum [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period	90 days	90 days
Real Estate Funds [Member] \| Separate Account Assets [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 244	$ 239
Hedge Funds [Member] \| Separate Account Assets [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	$ 0
Redemption Frequency	Up to Annually, varying by fund
Hedge Funds [Member] \| Separate Account Assets [Member] \| Minimum [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period	30 days	30 days
Hedge Funds [Member] \| Separate Account Assets [Member] \| Maximum [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period	90 days	90 days
Hedge Funds [Member] \| Separate Account Assets [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 51	$ 77

Fair value of investment in certain entities that calculate net asset value per share. Includes, but is not limited to, by unit, membership interest, or other ownership interest. Investment includes, but is not limited to, investment in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds. Excludes measurement of net asset value as estimate of fair value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

+ Details

Name:	us-gaap_AlternativeInvestmentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

States the frequency (for example, annually, semi-annually, quarterly) with which the entity can redeem its investment, or portion thereof, for each of those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments), by major category.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

+ Details

Name:	us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequency
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Notice period the entity is required to deliver before it can redeem an investment calculated using net asset value per share, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, investments calculated by per unit, per membership interest, other equity or ownership interest and alternative investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

+ Details

Name:	us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

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

No definition available.

+ Details

Name:	us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

+ Details

Name:	us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=ci_SeparateAccountAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_PartnershipInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_RealEstateFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_HedgeFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: