Change in discount rate assumption.

No definition available.

Name:	cinf_ChangeInDiscountRateAssumption
Namespace Prefix:	cinf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined Benefit Plan, Actual Percentage Return On Plan Assets

+ References

No definition available.

+ Details

Name:	cinf_DefinedBenefitPlanActualPercentageReturnOnPlanAssets
Namespace Prefix:	cinf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined Benefit Plan Company Shares Sold During Period

+ References

No definition available.

+ Details

Name:	cinf_DefinedBenefitPlanCompanySharesSoldDuringPeriod
Namespace Prefix:	cinf_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Age at which participants could elect to remain in the defined benefit plan as of August 31, 2008.

+ References

No definition available.

+ Details

Name:	cinf_DefinedBenefitPlanQualifyingAge
Namespace Prefix:	cinf_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined Contribution Plan, Requisite Service Period

+ References

No definition available.

+ Details

Name:	cinf_DefinedContributionPlanRequisiteServicePeriod
Namespace Prefix:	cinf_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying value as of the balance sheet date of obligations, excluding pension and other postretirement benefits, incurred through that date and payable for perquisites provided to employees pertaining to services received from them.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.15(a))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.15(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

The amount of employer securities or securities of parties related to the employer that are included in plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (n)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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

- Definition

Amount included in accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan expected to be recognized in net periodic benefit (cost) credit for fiscal year following most recent annual statement of financial position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (s)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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

- Definition

Amount included in accumulated other comprehensive (income) loss for prior service cost (credit) expected to be recognized in net periodic benefit cost (credit) for fiscal year following most recent annual statement of financial position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (s)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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

- Definition

Amount of contributions expected to be received by defined benefit plan from employer in next fiscal year following latest fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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

- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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

- Definition

The number of employer shares or related party shares included in plan assets.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanNumberOfSharesOfEquitySecuritiesIssuedByEmployerAndRelatedPartiesIncludedInPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Amount of discretionary contributions made by an employer to a defined contribution plan.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage employer matches of the employee's percentage contribution matched.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of paid and unpaid common stock dividends declared with the form of settlement in cash.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

Name:	us-gaap_DividendsCommonStockCash
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Value of the other investment not readily marketable determined by management based upon fair value methods, including pricing of similar securities and valuation techniques, that was used to record the investment for financial reporting purposes.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 820
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=35710923&loc=d3e45360-110995

+ Details

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

- Details

Name:	cinf_DefinedBenefitPlanByPlanAssetsSecondaryCategoriesAxis=cinf_DomesticEquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	cinf_DefinedBenefitPlanAssetAllocationsAxis=cinf_InformationSystemsAndTechnologyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	cinf_DefinedBenefitPlanAssetAllocationsAxis=us-gaap_FinancialServicesSectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	cinf_DefinedBenefitPlanAssetAllocationsAxis=cinf_HealthcareMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	cinf_DefinedBenefitPlanAssetAllocationsAxis=cinf_ConsumerDiscretionarySectorMemberDomain
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	cinf_DefinedBenefitPlanAssetAllocationsAxis=cinf_IndustrialSectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	cinf_DefinedBenefitPlanAssetAllocationsAxis=cinf_AllRemainingSectorsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	cinf_DefinedBenefitPlanByPlanAssetsSecondaryCategoriesAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	cinf_DefinedBenefitPlanByPlanAssetsSecondaryCategoriesAxis=us-gaap_DomesticCorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	cinf_DefinedBenefitPlanByPlanAssetsSecondaryCategoriesAxis=us-gaap_CashAndCashEquivalentsMember
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_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	cinf_DefinedBenefitPlanByBenefitPlanCategoriesAxis=cinf_BenefitObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_EquityFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanNameAxis=cinf_TopHatMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Employee Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2008	Dec. 31, 2018	Mar. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Minimum participant age to elect				40 years
Employer discretionary contribution amount (less than $1 million for 2016 and 2015 in the CFC Top Hat Savings Plan)	$ 16	$ 14	$ 12
Requisite service period	3 years
Accrued employee benefit		$ 15
Actual return on plan assets	17.40%	16.60%
Amortization of actuarial loss	$ 1
Amortization of prior service cost less than $1 million	1
Pension plan assets at fair value	$ 345	$ 315	278
Domestic Equity Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments	80.00%
Domestic Equity Securities \| Information Technology Sector
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments	22.00%
Domestic Equity Securities \| Financial Services Sector [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments	22.00%
Domestic Equity Securities \| Healthcare Sector
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments	15.00%
Domestic Equity Securities \| Consumer Discretionary Sector [Member] [Domain]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments	13.00%
Domestic Equity Securities \| Industrial Sector
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments	12.00%
Domestic Equity Securities \| All Remaining Sectors
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments	10.00%
States, Municipalities and Political Subdivisions
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments	9.00%
Domestic Corporate Debt Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Allocation percentage of investments	11.00%
Cash and Cash Equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Pension plan assets at fair value	$ 18	$ 11
Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Matching contribution to defined contribution plan	6.00%
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Percentage of increase decrease in discount rate	0.25%
Pension plan assets, shares	232,113	232,113
Pension plan assets, fair value	$ 17	$ 18
Shares sold during period		35,000
Cash dividends paid	$ 1	$ 1
Pension Plan \| Benefit Obligations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Percentage of increase decrease in discount rate	0.57%
Supplemental Employee Retirement Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Accrued employee benefit	$ 9
Percentage of increase decrease in discount rate	0.20%
Supplemental Employee Retirement Plan \| Benefit Obligations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Percentage of increase decrease in discount rate	0.49%
Top Hat Savings Plan - Mutual Funds and Common Equities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
CFC Top Hat Savings Plan	$ 31	24
Subsequent Event \| Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Expected contribution by employer during 2018						$ 5
Forecast \| Supplemental Employee Retirement Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Expected contribution by employer during 2018					$ 5
Top Hat \| Top Hat Savings Plan - Mutual Funds and Common Equities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Employer discretionary contribution amount (less than $1 million for 2016 and 2015 in the CFC Top Hat Savings Plan)	1	1	$ 1
CFC Top Hat Savings Plan	$ 31	$ 24