| **Fair Value Measurements (Fair Value of Note Payable and Long-Term Debt) (Detail) (USD $)In Millions, unless otherwise specified** | 12 Months Ended                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of note payable                                                                                                          | $ 49us-gaap_NotesPayableFairValueDisclosure                                                                                                                                                     | $ 104us-gaap_NotesPayableFairValueDisclosure |
| Total                                                                                                                               | 1,028us-gaap_DebtInstrumentFairValue                                                                                                                                                            | 991us-gaap_DebtInstrumentFairValue |
| 6.900% Senior Debentures, Due 2028                                                                                                  |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of senior debt                                                                                                           | 34us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures69PercentDue2028Member                                                                                        | 30us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures69PercentDue2028Member |
| Interest rate                                                                                                                       | 6.90%us-gaap_DebtInstrumentInterestRateStatedPercentage/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures69PercentDue2028Member                                                                | 6.90%us-gaap_DebtInstrumentInterestRateStatedPercentage/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures69PercentDue2028Member |
| Debt instrument, year of maturity                                                                                                   | 2028                                                                                                                                                                                            | 2028 |
| 6.920% Senior Debentures, Due 2028                                                                                                  |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of senior debt                                                                                                           | 496us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures692PercentDue2028Member                                                                                      | 458us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures692PercentDue2028Member |
| Interest rate                                                                                                                       | 6.92%us-gaap_DebtInstrumentInterestRateStatedPercentage/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures692PercentDue2028Member                                                               | 6.92%us-gaap_DebtInstrumentInterestRateStatedPercentage/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures692PercentDue2028Member |
| Debt instrument, year of maturity                                                                                                   | 2028                                                                                                                                                                                            | 2028 |
| 6.125% Senior Notes, Due 2034                                                                                                       |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of senior debt                                                                                                           | 449us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorNotes6125PercentDue2034Member                                                                                          | 399us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorNotes6125PercentDue2034Member |
| Interest rate                                                                                                                       | 6.125%us-gaap_DebtInstrumentInterestRateStatedPercentage/ us-gaap_DebtInstrumentAxis= cinf_SeniorNotes6125PercentDue2034Member                                                                  | 6.125%us-gaap_DebtInstrumentInterestRateStatedPercentage/ us-gaap_DebtInstrumentAxis= cinf_SeniorNotes6125PercentDue2034Member |
| Debt instrument, year of maturity                                                                                                   | 2034                                                                                                                                                                                            | 2034 |
| Level 1                                                                                                                             |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of note payable                                                                                                          | 0us-gaap_NotesPayableFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                   | 0us-gaap_NotesPayableFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member |
| Total                                                                                                                               | 0us-gaap_DebtInstrumentFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                           | 0us-gaap_DebtInstrumentFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member |
| Level 1 | 6.900% Senior Debentures, Due 2028                                                                                        |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of senior debt                                                                                                           | 0us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures69PercentDue2028Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member    | 0us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures69PercentDue2028Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member |
| Level 1 | 6.920% Senior Debentures, Due 2028                                                                                        |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of senior debt                                                                                                           | 0us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures692PercentDue2028Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member   | 0us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures692PercentDue2028Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member |
| Level 1 | 6.125% Senior Notes, Due 2034                                                                                             |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of senior debt                                                                                                           | 0us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorNotes6125PercentDue2034Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member       | 0us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorNotes6125PercentDue2034Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member |
| Level 2                                                                                                                             |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of note payable                                                                                                          | 49us-gaap_NotesPayableFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                  | 104us-gaap_NotesPayableFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member |
| Total                                                                                                                               | 1,028us-gaap_DebtInstrumentFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                       | 991us-gaap_DebtInstrumentFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member |
| Level 2 | 6.900% Senior Debentures, Due 2028                                                                                        |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of senior debt                                                                                                           | 34us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures69PercentDue2028Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member   | 30us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures69PercentDue2028Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member |
| Level 2 | 6.920% Senior Debentures, Due 2028                                                                                        |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of senior debt                                                                                                           | 496us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures692PercentDue2028Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member | 458us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures692PercentDue2028Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member |
| Level 2 | 6.125% Senior Notes, Due 2034                                                                                             |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of senior debt                                                                                                           | 449us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorNotes6125PercentDue2034Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member     | 399us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorNotes6125PercentDue2034Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member |
| Level 3                                                                                                                             |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of note payable                                                                                                          | 0us-gaap_NotesPayableFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                   | 0us-gaap_NotesPayableFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member |
| Total                                                                                                                               | 0us-gaap_DebtInstrumentFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                           | 0us-gaap_DebtInstrumentFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member |
| Level 3 | 6.900% Senior Debentures, Due 2028                                                                                        |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of senior debt                                                                                                           | 0us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures69PercentDue2028Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member    | 0us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures69PercentDue2028Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member |
| Level 3 | 6.920% Senior Debentures, Due 2028                                                                                        |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of senior debt                                                                                                           | 0us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures692PercentDue2028Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member   | 0us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorDebentures692PercentDue2028Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member |
| Level 3 | 6.125% Senior Notes, Due 2034                                                                                             |                                                                                                                                                                                            |   |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                  |                                                                                                                                                                                            |   |
| Fair value of senior debt                                                                                                           | $ 0us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorNotes6125PercentDue2034Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member     | $ 0us-gaap_LongTermDebtFairValue/ us-gaap_DebtInstrumentAxis= cinf_SeniorNotes6125PercentDue2034Member/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13476-108611
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Contractual interest rate for funds borrowed, under the debt agreement.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_DebtInstrumentInterestRateStatedPercentage**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Description of the maturity date of the debt instrument including whether the debt matures serially and, if so, a brief description of the serial maturities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(2)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_DebtInstrumentMaturityDateDescription**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.
+ References
No definition available.
+ Details**Name:**us-gaap_LongTermDebtFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of notes payable.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19,20,22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_NotesPayableFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DebtInstrumentAxis=cinf_SeniorDebentures692PercentDue2028Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DebtInstrumentAxis=cinf_SeniorNotes6125PercentDue2034Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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