Employee Retirement Benefits (Components Of Net Periodic Benefit Cost As Well As Other Changes In Plan Assets And Benefit Obligations Recognized In Other Comprehensive Income For Qualified Plan And SERP) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Compensation and Retirement Disclosure [Abstract]
Service cost	$ 10us-gaap_DefinedBenefitPlanServiceCost	$ 13us-gaap_DefinedBenefitPlanServiceCost	$ 12us-gaap_DefinedBenefitPlanServiceCost
Interest cost	15us-gaap_DefinedBenefitPlanInterestCost	13us-gaap_DefinedBenefitPlanInterestCost	14us-gaap_DefinedBenefitPlanInterestCost
Expected return on plan assets	(17)	(17)	(16)
Amortization of actuarial loss and prior service cost	2	9	8
Other	3	2	0
Net periodic benefit cost	13	20	18
Current year actuarial loss (gain)	21	(72)	20
Amortization of actuarial loss	5	10	6
Other	2us-gaap_DefinedBenefitPlanOtherCosts	0us-gaap_DefinedBenefitPlanOtherCosts	0us-gaap_DefinedBenefitPlanOtherCosts
Amortization of prior service cost	0	(1)	(1)
Total recognized in other comprehensive loss (income)	$ 18	$ (83)	$ 13

Employee Retirement Benefits (Components Of Net Periodic Benefit Cost As Well As Other Changes In Plan Assets And Benefit Obligations Recognized In Other Comprehensive Income For Qualified Plan And SERP) (Details) (USD $)
In Millions, unless otherwise specified

12 Months Ended

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Compensation and Retirement Disclosure [Abstract]

Service cost

$ 10us-gaap_DefinedBenefitPlanServiceCost

$ 13us-gaap_DefinedBenefitPlanServiceCost

$ 12us-gaap_DefinedBenefitPlanServiceCost

Interest cost

15us-gaap_DefinedBenefitPlanInterestCost

13us-gaap_DefinedBenefitPlanInterestCost

14us-gaap_DefinedBenefitPlanInterestCost

Expected return on plan assets

(17)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets

(16)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets

Amortization of actuarial loss and prior service cost

2cinf_DefinedBenefitPlanAmortizationOfNetActuarialGainLossAndPriorServiceCostOrBenefit

9cinf_DefinedBenefitPlanAmortizationOfNetActuarialGainLossAndPriorServiceCostOrBenefit

8cinf_DefinedBenefitPlanAmortizationOfNetActuarialGainLossAndPriorServiceCostOrBenefit

Other

3cinf_DefinedBenefitPlanOtherNetPeriodicBenefitCosts

2cinf_DefinedBenefitPlanOtherNetPeriodicBenefitCosts

0cinf_DefinedBenefitPlanOtherNetPeriodicBenefitCosts

Net periodic benefit cost

13us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost

20us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost

18us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost

Current year actuarial loss (gain)

21us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax

(72)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax

20us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax

Amortization of actuarial loss

5us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax

10us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax

6us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax

Other

2us-gaap_DefinedBenefitPlanOtherCosts

0us-gaap_DefinedBenefitPlanOtherCosts

Amortization of prior service cost

0us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax

(1)us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax

Total recognized in other comprehensive loss (income)

$ 18us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax

$ (83)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax

$ 13us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax

- Definition

The amount of actuarial gains or losses and prior service cost or credit recognized in net periodic benefit cost.

+ References

No definition available.

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- Definition

Defined Benefit Plan, Other Net Periodic Benefit Costs

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- Definition

Amount before tax, after reclassification adjustments, of prior service cost (credit) that has not been recognized in the income statement.

+ References

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- Definition

An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Glossary Expected Return on Plan Assets

-URI http://asc.fasb.org/extlink&oid=6512136

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- Definition

The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

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- Definition

The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.

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- Definition

Amount of other defined benefit plan costs.

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No definition available.

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- Definition

The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

+ 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 (h)(1)

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- Definition

Amount before tax, after reclassification adjustments, of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans.

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-Topic 220

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- Definition

Amount before tax of (increase) decrease in the value of the projected benefit obligation related to a change in an actuarial assumption and increase (decrease) in the value of the plan assets resulting from experience different from that assumed, that has not been recognized in net periodic benefit (cost) credit.

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- Definition

Amount before tax of reclassification adjustment from accumulated other comprehensive income (loss) for actuarial gain (loss) related to pension and other postretirement benefit plans.

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