Fair Value Measurements - Separate Account Assets Priced at NAV (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	$ 57
Redemption Frequency	quarterly redemption frequency
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]
Fair Value	$ 217	$ 239
Separate Account Assets [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	360
Separate Account Assets [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Fair Value	724	732
Security Partnerships [Member] \| Separate Account Assets [Member]
Fair Value Investments, Entities That Calculate Net Asset Value Per Share [Line Items]
Unfunded Commitments	$ 360
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	$ 494	477
Expected liquidation period after inception	10 years
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	30 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
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	$ 218	237
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
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
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	$ 12	$ 18

Expected liquidation period after inception of the underlying assets measured at net asset value per share or unit.

No definition available.

Name:	ci_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareLiquidatingInvestmentPeriodFromInception
Namespace Prefix:	ci_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 35
-Paragraph 54B
-URI http://asc.fasb.org/extlink&oid=99392525&loc=SL7495116-110257

+ Details

Name:	us-gaap_AlternativeInvestment
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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=99377789&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

+ 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

+ Details

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

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_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=srt_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: