| **Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) (USD $)In Millions, unless otherwise specified** | Mar. 31, 2014 | Dec. 31, 2013 |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Debt Securities                                                                                                                                                                |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | $ 8,609       | $ 8,638       |
| Gross Unrealized Gains                                                                                                                                                         | 613           | 558           |
| Gross Unrealized Losses                                                                                                                                                        | 42            | 75            |
| Fair Value                                                                                                                                                                     | 9,180         | 9,121         |
| Debt Securities | States, Municipalities And Political Subdivisions                                                                                                            |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 3,113         | 3,107         |
| Gross Unrealized Gains                                                                                                                                                         | 143           | 125           |
| Gross Unrealized Losses                                                                                                                                                        | 12            | 21            |
| Fair Value                                                                                                                                                                     | 3,244         | 3,211         |
| Debt Securities | Convertibles And Bonds With Warrants Attached                                                                                                                |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 17            | 17            |
| Gross Unrealized Gains                                                                                                                                                         | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                                        | 0             | 0             |
| Fair Value                                                                                                                                                                     | 17            | 17            |
| Debt Securities | United States Government                                                                                                                                     |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 7             | 7             |
| Gross Unrealized Gains                                                                                                                                                         | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                                        | 0             | 0             |
| Fair Value                                                                                                                                                                     | 7             | 7             |
| Debt Securities | Government-Sponsored Enterprises                                                                                                                             |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 211           | 227           |
| Gross Unrealized Gains                                                                                                                                                         | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                                        | 20            | 27            |
| Fair Value                                                                                                                                                                     | 191           | 200           |
| Debt Securities | Foreign Government                                                                                                                                           |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 10            | 10            |
| Gross Unrealized Gains                                                                                                                                                         | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                                        | 0             | 0             |
| Fair Value                                                                                                                                                                     | 10            | 10            |
| Debt Securities | Commercial Mortgage Backed Securities                                                                                                                        |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 206           | 148           |
| Gross Unrealized Gains                                                                                                                                                         | 2             | 0             |
| Gross Unrealized Losses                                                                                                                                                        | 2             | 5             |
| Fair Value                                                                                                                                                                     | 206           | 143           |
| Debt Securities | Corporate Securities                                                                                                                                         |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 5,045         | 5,122         |
| Gross Unrealized Gains                                                                                                                                                         | 468           | 433           |
| Gross Unrealized Losses                                                                                                                                                        | 8             | 22            |
| Fair Value                                                                                                                                                                     | 5,505         | 5,533         |
| Equity Securities                                                                                                                                                              |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 2,545         | 2,523         |
| Gross Unrealized Gains                                                                                                                                                         | 1,888         | 1,856         |
| Gross Unrealized Losses                                                                                                                                                        | 7             | 4             |
| Fair Value                                                                                                                                                                     | 4,426         | 4,375         |
| Equity Securities | Common Stock                                                                                                                                               |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 2,418         | 2,396         |
| Gross Unrealized Gains                                                                                                                                                         | 1,845         | 1,818         |
| Gross Unrealized Losses                                                                                                                                                        | 6             | 1             |
| Fair Value                                                                                                                                                                     | 4,257         | 4,213         |
| Equity Securities | Preferred Stock                                                                                                                                            |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 127           | 127           |
| Gross Unrealized Gains                                                                                                                                                         | 43            | 38            |
| Gross Unrealized Losses                                                                                                                                                        | 1             | 3             |
| Fair Value                                                                                                                                                                     | 169           | 162           |
| Fixed Maturities And Equity Securities                                                                                                                                         |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 11,154        | 11,161        |
| Gross Unrealized Gains                                                                                                                                                         | 2,501         | 2,414         |
| Gross Unrealized Losses                                                                                                                                                        | 49            | 79            |
| Fair Value                                                                                                                                                                     | $ 13,606      | $ 13,496      |

| 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 |