| **Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) (USD $)In Millions, unless otherwise specified** | Sep. 30, 2013 | Dec. 31, 2012 |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Debt Securities                                                                                                                                                                |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | $ 8,502       | $ 8,222       |
| Gross Unrealized Gains                                                                                                                                                         | 600           | 874           |
| Gross Unrealized Losses                                                                                                                                                        | 64            | 3             |
| Fair Value                                                                                                                                                                     | 9,038         | 9,093         |
| Debt Securities | States, Municipalities And Political Subdivisions                                                                                                            |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 3,046         | 3,040         |
| Gross Unrealized Gains                                                                                                                                                         | 146           | 250           |
| Gross Unrealized Losses                                                                                                                                                        | 17            | 1             |
| Fair Value                                                                                                                                                                     | 3,175         | 3,289         |
| Debt Securities | Convertibles And Bonds With Warrants Attached                                                                                                                |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 16            | 31            |
| Gross Unrealized Gains                                                                                                                                                         | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                                        | 0             | 0             |
| Fair Value                                                                                                                                                                     | 16            | 31            |
| Debt Securities | United States Government                                                                                                                                     |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 7             | 7             |
| Gross Unrealized Gains                                                                                                                                                         | 0             | 1             |
| Gross Unrealized Losses                                                                                                                                                        | 0             | 0             |
| Fair Value                                                                                                                                                                     | 7             | 8             |
| Debt Securities | Government-Sponsored Enterprises                                                                                                                             |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 209           | 164           |
| Gross Unrealized Gains                                                                                                                                                         | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                                        | 19            | 0             |
| Fair Value                                                                                                                                                                     | 190           | 164           |
| Debt Securities | Foreign Government                                                                                                                                           |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 10            | 3             |
| Gross Unrealized Gains                                                                                                                                                         | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                                        | 0             | 0             |
| Fair Value                                                                                                                                                                     | 10            | 3             |
| Debt Securities | Commercial Mortgage Backed Securities                                                                                                                        |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 79            | 27            |
| Gross Unrealized Gains                                                                                                                                                         | 0             | 1             |
| Gross Unrealized Losses                                                                                                                                                        | 3             | 0             |
| Fair Value                                                                                                                                                                     | 76            | 28            |
| Debt Securities | Corporate Securities                                                                                                                                         |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 5,135         | 4,950         |
| Gross Unrealized Gains                                                                                                                                                         | 454           | 622           |
| Gross Unrealized Losses                                                                                                                                                        | 25            | 2             |
| Fair Value                                                                                                                                                                     | 5,564         | 5,570         |
| Equity Securities                                                                                                                                                              |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 2,471         | 2,369         |
| Gross Unrealized Gains                                                                                                                                                         | 1,516         | 1,014         |
| Gross Unrealized Losses                                                                                                                                                        | 3             | 10            |
| Fair Value                                                                                                                                                                     | 3,984         | 3,373         |
| Equity Securities | Common Stock                                                                                                                                               |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 2,394         | 2,270         |
| Gross Unrealized Gains                                                                                                                                                         | 1,482         | 977           |
| Gross Unrealized Losses                                                                                                                                                        | 3             | 9             |
| Fair Value                                                                                                                                                                     | 3,873         | 3,238         |
| Equity Securities | Preferred Stock                                                                                                                                            |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 77            | 99            |
| Gross Unrealized Gains                                                                                                                                                         | 34            | 37            |
| Gross Unrealized Losses                                                                                                                                                        | 0             | 1             |
| Fair Value                                                                                                                                                                     | 111           | 135           |
| Fixed Maturities And Equity Securities                                                                                                                                         |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 10,973        | 10,591        |
| Gross Unrealized Gains                                                                                                                                                         | 2,116         | 1,888         |
| Gross Unrealized Losses                                                                                                                                                        | 67            | 13            |
| Fair Value                                                                                                                                                                     | $ 13,022      | $ 12,466      |

| X                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.
+ References
No definition available.
+ Details**Name:**cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.
+ References
No definition available.
+ Details**Name:**cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562
+ Details**Name:**us-gaap_AvailableForSaleSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAmortizedCost**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |