| **Investments (Fair Values And Unrealized Losses By Investment Category And By The Duration Of The Securities' Continuous Unrealized Loss Position) (Details) - USD ($) $ in Millions** | Dec. 31, 2015 | Dec. 31, 2014 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Investments, Unrealized Loss Position**                                                                                                                                               |          |          |
| Less than 12 months fair value                                                                                                                                                          | $ 1,654       | $ 393         |
| Less than 12 months unrealized losses                                                                                                                                                   | 119           | 11            |
| 12 months or more fair value                                                                                                                                                            | 284           | 446           |
| 12 months or more unrealized losses                                                                                                                                                     | 37            | 11            |
| Total fair value                                                                                                                                                                        | 1,938         | 839           |
| Total unrealized losses                                                                                                                                                                 | 156           | 22            |
| Fixed Maturities                                                                                                                                                                        |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                               |          |          |
| Less than 12 months fair value                                                                                                                                                          | 1,349         | 292           |
| Less than 12 months unrealized losses                                                                                                                                                   | 68            | 8             |
| 12 months or more fair value                                                                                                                                                            | 284           | 429           |
| 12 months or more unrealized losses                                                                                                                                                     | 37            | 10            |
| Total fair value                                                                                                                                                                        | 1,633         | 721           |
| Total unrealized losses                                                                                                                                                                 | 105           | 18            |
| Fixed Maturities | Corporate Securities                                                                                                                                                 |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                               |          |          |
| Less than 12 months fair value                                                                                                                                                          | 1,099         | 261           |
| Less than 12 months unrealized losses                                                                                                                                                   | 63            | 8             |
| 12 months or more fair value                                                                                                                                                            | 133           | 90            |
| 12 months or more unrealized losses                                                                                                                                                     | 33            | 3             |
| Total fair value                                                                                                                                                                        | 1,232         | 351           |
| Total unrealized losses                                                                                                                                                                 | 96            | 11            |
| Fixed Maturities | States, Municipalities and Political Subdivisions                                                                                                                    |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                               |          |          |
| Less than 12 months fair value                                                                                                                                                          | 47            | 17            |
| Less than 12 months unrealized losses                                                                                                                                                   | 1             | 0             |
| 12 months or more fair value                                                                                                                                                            | 22            | 135           |
| 12 months or more unrealized losses                                                                                                                                                     | 0             | 2             |
| Total fair value                                                                                                                                                                        | 69            | 152           |
| Total unrealized losses                                                                                                                                                                 | 1             | 2             |
| Fixed Maturities | Commercial Mortgage Backed Securities                                                                                                                                |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                               |          |          |
| Less than 12 months fair value                                                                                                                                                          | 103           | 3             |
| Less than 12 months unrealized losses                                                                                                                                                   | 2             | 0             |
| 12 months or more fair value                                                                                                                                                            | 2             | 23            |
| 12 months or more unrealized losses                                                                                                                                                     | 0             | 0             |
| Total fair value                                                                                                                                                                        | 105           | 26            |
| Total unrealized losses                                                                                                                                                                 | 2             | 0             |
| Fixed Maturities | Government-Sponsored Enterprises                                                                                                                                     |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                               |          |          |
| Less than 12 months fair value                                                                                                                                                          | 100           | 11            |
| Less than 12 months unrealized losses                                                                                                                                                   | 2             | 0             |
| 12 months or more fair value                                                                                                                                                            | 127           | 181           |
| 12 months or more unrealized losses                                                                                                                                                     | 4             | 5             |
| Total fair value                                                                                                                                                                        | 227           | 192           |
| Total unrealized losses                                                                                                                                                                 | 6             | 5             |
| Equity Securities                                                                                                                                                                       |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                               |          |          |
| Less than 12 months fair value                                                                                                                                                          | 305           | 101           |
| Less than 12 months unrealized losses                                                                                                                                                   | 51            | 3             |
| 12 months or more fair value                                                                                                                                                            | 0             | 17            |
| 12 months or more unrealized losses                                                                                                                                                     | 0             | 1             |
| Total fair value                                                                                                                                                                        | 305           | 118           |
| Total unrealized losses                                                                                                                                                                 | 51            | 4             |
| Equity Securities | Common Equities                                                                                                                                                     |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                               |          |          |
| Less than 12 months fair value                                                                                                                                                          | 270           | 85            |
| Less than 12 months unrealized losses                                                                                                                                                   | 51            | 3             |
| 12 months or more fair value                                                                                                                                                            | 0             | 0             |
| 12 months or more unrealized losses                                                                                                                                                     | 0             | 0             |
| Total fair value                                                                                                                                                                        | 270           | 85            |
| Total unrealized losses                                                                                                                                                                 | 51            | 3             |
| Equity Securities | Nonredeemable Preferred Equities                                                                                                                                    |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                               |          |          |
| Less than 12 months fair value                                                                                                                                                          | 35            | 16            |
| Less than 12 months unrealized losses                                                                                                                                                   | 0             | 0             |
| 12 months or more fair value                                                                                                                                                            | 0             | 17            |
| 12 months or more unrealized losses                                                                                                                                                     | 0             | 1             |
| Total fair value                                                                                                                                                                        | 35            | 33            |
| Total unrealized losses                                                                                                                                                                 | $ 0           | $ 1           |

| X                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized loss on securities classified as available-for-sale that have been in a loss position for twelve months or longer.
+ References
No definition available.
+ Details**Name:**cinf_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLoss3**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized loss on securities classified as available-for-sale.
+ References
No definition available.
+ Details**Name:**cinf_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAggregateLoss3**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized loss on investments in available-for-sale securities that have been in a loss position for less than twelve months.
+ References
No definition available.
+ Details**Name:**cinf_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLoss3**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Investments, Unrealized Loss Position [Line Items]
+ References
No definition available.
+ Details**Name:**cinf_InvestmentsUnrealizedLossPositionLineItems**Namespace Prefix:**cinf_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**invest_InvestmentSecuritiesClassAxis=us-gaap_DebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CorporateBondSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CommercialMortgageBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**invest_InvestmentSecuritiesClassAxis=us-gaap_EquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CommonStockMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_PreferredStockMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |