| **Property Casualty Loss and Loss Expenses  - Additional Information (Detail) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 9 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                                        |           |           |   |   |
| Favorable reserve development on prior accident years                                                                          | $ 38           | $ 86           | $ 140 | $ 287 |
| Life and Health Loss Reserves                                                                                                  |           |           |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                                        |           |           |   |   |
| Reserve for loss and loss expenses                                                                                             | 69             | 64             | 69 | 64 |
| Commercial Lines                                                                                                               |           |           |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                                        |           |           |   |   |
| Favorable reserve development on prior accident years                                                                          | 13             |           | 91 |   |
| Commercial Lines | Catastrophe Development                                                                                     |           |           |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                                        |           |           |   |   |
| Favorable reserve development on prior accident years                                                                          | 9              | 12             | 24 | 39 |
| Personal Lines                                                                                                                 |           |           |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                                        |           |           |   |   |
| Favorable reserve development on prior accident years                                                                          | 21             |           | 43 |   |
| Excess And Surplus Lines Insurance                                                                                             |           |           |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                                        |           |           |   |   |
| Favorable reserve development on prior accident years                                                                          | $ 4            |           | $ 6 |   |

| X                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recovery of Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years
+ References
No definition available.
+ Details**Name:**cinf_RecoveryOfLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a),(b),(c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437
+ Details**Name:**us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |