| **Investments (Analysis Of Cost Or Amortized Cost Gross Unrealized Gains And Losses And Fair Value For Investments) (Details) - USD ($) $ in Millions** | Dec. 31, 2015 | Dec. 31, 2014 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Fixed Maturities                                                                                                                                        |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                           |          |          |
| Cost or amortized cost                                                                                                                                  | $ 9,324       | $ 8,871       |
| Gross unrealized gains                                                                                                                                  | 431           | 607           |
| Gross unrealized losses                                                                                                                                 | 105           | 18            |
| Fair value                                                                                                                                              | 9,650         | 9,460         |
| Fixed Maturities | Corporate Securities                                                                                                                 |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                           |          |          |
| Cost or amortized cost                                                                                                                                  | 5,294         | 5,117         |
| Gross unrealized gains                                                                                                                                  | 255           | 420           |
| Gross unrealized losses                                                                                                                                 | 96            | 11            |
| Fair value                                                                                                                                              | 5,453         | 5,526         |
| Fixed Maturities | States, Municipalities and Political Subdivisions                                                                                    |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                           |          |          |
| Cost or amortized cost                                                                                                                                  | 3,440         | 3,267         |
| Gross unrealized gains                                                                                                                                  | 172           | 178           |
| Gross unrealized losses                                                                                                                                 | 1             | 2             |
| Fair value                                                                                                                                              | 3,611         | 3,443         |
| Fixed Maturities | Commercial Mortgage Backed Securities                                                                                                |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                           |          |          |
| Cost or amortized cost                                                                                                                                  | 287           | 250           |
| Gross unrealized gains                                                                                                                                  | 4             | 9             |
| Gross unrealized losses                                                                                                                                 | 2             | 0             |
| Fair value                                                                                                                                              | 289           | 259           |
| Fixed Maturities | Government-Sponsored Enterprises                                                                                                     |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                           |          |          |
| Cost or amortized cost                                                                                                                                  | 284           | 213           |
| Gross unrealized gains                                                                                                                                  | 0             | 0             |
| Gross unrealized losses                                                                                                                                 | 6             | 5             |
| Fair value                                                                                                                                              | 278           | 208           |
| Fixed Maturities | Foreign Government                                                                                                                   |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                           |          |          |
| Cost or amortized cost                                                                                                                                  | 10            | 10            |
| Gross unrealized gains                                                                                                                                  | 0             | 0             |
| Gross unrealized losses                                                                                                                                 | 0             | 0             |
| Fair value                                                                                                                                              | 10            | 10            |
| Fixed Maturities | Convertibles and Bonds with Warrants Attached                                                                                        |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                           |          |          |
| Cost or amortized cost                                                                                                                                  | 5             | 7             |
| Gross unrealized gains                                                                                                                                  | 0             | 0             |
| Gross unrealized losses                                                                                                                                 | 0             | 0             |
| Fair value                                                                                                                                              | 5             | 7             |
| Fixed Maturities | United States Government                                                                                                             |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                           |          |          |
| Cost or amortized cost                                                                                                                                  | 4             | 7             |
| Gross unrealized gains                                                                                                                                  | 0             | 0             |
| Gross unrealized losses                                                                                                                                 | 0             | 0             |
| Fair value                                                                                                                                              | 4             | 7             |
| Equity Securities                                                                                                                                       |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                           |          |          |
| Cost or amortized cost                                                                                                                                  | 2,938         | 2,728         |
| Gross unrealized gains                                                                                                                                  | 1,819         | 2,134         |
| Gross unrealized losses                                                                                                                                 | 51            | 4             |
| Fair value                                                                                                                                              | 4,706         | 4,858         |
| Equity Securities | Common Equities                                                                                                                     |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                           |          |          |
| Cost or amortized cost                                                                                                                                  | 2,749         | 2,583         |
| Gross unrealized gains                                                                                                                                  | 1,787         | 2,099         |
| Gross unrealized losses                                                                                                                                 | 51            | 3             |
| Fair value                                                                                                                                              | 4,485         | 4,679         |
| Equity Securities | Nonredeemable Preferred Equities                                                                                                    |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                           |          |          |
| Cost or amortized cost                                                                                                                                  | 189           | 145           |
| Gross unrealized gains                                                                                                                                  | 32            | 35            |
| Gross unrealized losses                                                                                                                                 | 0             | 1             |
| Fair value                                                                                                                                              | 221           | 179           |
| Fixed Maturities And Equity Securities                                                                                                                  |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                           |          |          |
| Cost or amortized cost                                                                                                                                  | 12,262        | 11,599        |
| Gross unrealized gains                                                                                                                                  | 2,250         | 2,741         |
| Gross unrealized losses                                                                                                                                 | 156           | 22            |
| Fair value                                                                                                                                              | $ 14,356      | $ 14,318      |

| 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.
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| X                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
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| 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
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+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAmortizedCost**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=cinf_ForeignGovernmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                              |
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| X                                                                                                                                                                                        |
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                    |
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| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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