| **Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) (USD $)In Millions, unless otherwise specified** | Jun. 30, 2014 | Dec. 31, 2013 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | $ 164         | $ 1,516       |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 1             | 74            |
| 12 months or more Fair Value                                                                                                                                                                          | 708           | 86            |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 25            | 5             |
| Total Fair Value                                                                                                                                                                                      | 872           | 1,602         |
| Total Unrealized Losses                                                                                                                                                                               | 26            | 79            |
| Fixed Maturities                                                                                                                                                                                      |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 120           | 1,397         |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 1             | 70            |
| 12 months or more Fair Value                                                                                                                                                                          | 612           | 86            |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 21            | 5             |
| Total Fair Value                                                                                                                                                                                      | 732           | 1,483         |
| Total Unrealized Losses                                                                                                                                                                               | 22            | 75            |
| Fixed Maturities | States, Municipalities And Political Subdivisions                                                                                                                                  |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 61            | 490           |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 1             | 18            |
| 12 months or more Fair Value                                                                                                                                                                          | 267           | 42            |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 4             | 3             |
| Total Fair Value                                                                                                                                                                                      | 328           | 532           |
| Total Unrealized Losses                                                                                                                                                                               | 5             | 21            |
| Fixed Maturities | United States Government                                                                                                                                                           |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 0             | 1             |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 0             | 0             |
| 12 months or more Fair Value                                                                                                                                                                          | 1             | 0             |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             | 0             |
| Total Fair Value                                                                                                                                                                                      | 1             | 1             |
| Total Unrealized Losses                                                                                                                                                                               | 0             | 0             |
| Fixed Maturities | Government-Sponsored Enterprises                                                                                                                                                   |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 10            | 199           |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 0             | 27            |
| 12 months or more Fair Value                                                                                                                                                                          | 175           | 1             |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 13            | 0             |
| Total Fair Value                                                                                                                                                                                      | 185           | 200           |
| Total Unrealized Losses                                                                                                                                                                               | 13            | 27            |
| Fixed Maturities | Foreign Government Debt Securities [Member]                                                                                                                                        |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 0             | 10            |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 0             | 0             |
| 12 months or more Fair Value                                                                                                                                                                          | 10            | 0             |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 0             | 0             |
| Total Fair Value                                                                                                                                                                                      | 10            | 10            |
| Total Unrealized Losses                                                                                                                                                                               | 0             | 0             |
| Fixed Maturities | Commercial Mortgage Backed Securities                                                                                                                                              |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 0             | 125           |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 0             | 5             |
| 12 months or more Fair Value                                                                                                                                                                          | 32            | 0             |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 1             | 0             |
| Total Fair Value                                                                                                                                                                                      | 32            | 125           |
| Total Unrealized Losses                                                                                                                                                                               | 1             | 5             |
| Fixed Maturities | Corporate Securities                                                                                                                                                               |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 49            | 572           |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 0             | 20            |
| 12 months or more Fair Value                                                                                                                                                                          | 127           | 43            |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 3             | 2             |
| Total Fair Value                                                                                                                                                                                      | 176           | 615           |
| Total Unrealized Losses                                                                                                                                                                               | 3             | 22            |
| Equity Securities                                                                                                                                                                                     |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 44            | 119           |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 0             | 4             |
| 12 months or more Fair Value                                                                                                                                                                          | 96            | 0             |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 4             | 0             |
| Total Fair Value                                                                                                                                                                                      | 140           | 119           |
| Total Unrealized Losses                                                                                                                                                                               | 4             | 4             |
| Equity Securities | Common Stock                                                                                                                                                                      |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 39            | 77            |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 0             | 1             |
| 12 months or more Fair Value                                                                                                                                                                          | 79            | 0             |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 3             | 0             |
| Total Fair Value                                                                                                                                                                                      | 118           | 77            |
| Total Unrealized Losses                                                                                                                                                                               | 3             | 1             |
| Equity Securities | Preferred Stock                                                                                                                                                                   |          |          |
| **Investments, Unrealized Loss Position [Line Items]**                                                                                                                                                |          |          |
| Less than 12 months Fair Value                                                                                                                                                                        | 5             | 42            |
| Less than 12 months Unrealized Losses                                                                                                                                                                 | 0             | 3             |
| 12 months or more Fair Value                                                                                                                                                                          | 17            | 0             |
| 12 months or more Unrealized Losses                                                                                                                                                                   | 1             | 0             |
| Total Fair Value                                                                                                                                                                                      | 22            | 42            |
| Total Unrealized Losses                                                                                                                                                                               | $ 1           | $ 3           |

| 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                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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 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_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |