| **Property Casualty Loss And Loss Expenses (Narrative) (Details) - USD ($)**                      | 12 Months Ended |                 |
|---------------------------------------------------------------------------------------------------|-----------------|-----------------|
||
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**           |            |            |   |   |
| Certain accident, life and health loss reserves                                                   | $ 4,660,000,000 | $ 4,438,000,000 | $ 4,241,000,000 | $ 4,169,000,000 |
| Prior year claims and claims adjustment expense                                                   | (184,000,000)   | (98,000,000)    | (147,000,000) |   |
| Recovery Of Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims Prior Years | (86,000,000)    |            |   |   |
| Increase in favorable reserve developement on prior accident                                      |            | 49,000,000      | 249,000,000 |   |
| Net loss and loss expense reserves for asbestos and environmental claims                          | 84,000,000      | 81,000,000      |   |   |
| Environmental claims related to mold                                                              | $ 45,000,000    | $ 51,000,000    |   |   |
| Percentage of reserves for asbestos and environmental claims                                      | 1.90%           | 2.00%           |   |   |
| Maximum                                                                                           |            |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**           |            |            |   |   |
| Reinsurance retention                                                                             | $ 500,000       |            |   |   |
| Certain Accident, Life and Health Lines                                                           |            |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**           |            |            |   |   |
| Certain accident, life and health loss reserves                                                   | 58,000,000      | $ 47,000,000    | 70,000,000 |   |
| Commercial Insurance                                                                              |            |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**           |            |            |   |   |
| Prior year claims and claims adjustment expense                                                   | (154,000,000)   | (57,000,000)    | (95,000,000) |   |
| Commercial Insurance | Workers' Compensation Line                                                 |            |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**           |            |            |   |   |
| Prior year claims and claims adjustment expense                                                   | (93,000,000)    | (51,000,000)    | (14,000,000) |   |
| Commercial Insurance | Commercial Property Line                                                   |            |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**           |            |            |   |   |
| Prior year claims and claims adjustment expense                                                   |            | (34,000,000)    | (22,000,000) |   |
| Commercial Insurance | Homeowner Line                                                             |            |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**           |            |            |   |   |
| Prior year claims and claims adjustment expense                                                   |            | (14,000,000)    | (19,000,000) |   |
| Commercial Insurance | Commercial Casualty Line                                                   |            |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**           |            |            |   |   |
| Prior year claims and claims adjustment expense                                                   | (63,000,000)    | 5,000,000       | (70,000,000) |   |
| Commercial Insurance | Commercial Auto Line                                                       |            |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**           |            |            |   |   |
| Prior year claims and claims adjustment expense                                                   | 31,000,000      | 39,000,000      |   |   |
| Personal Insurance                                                                                |            |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**           |            |            |   |   |
| Prior year claims and claims adjustment expense                                                   | 5,000,000       | (12,000,000)    | (39,000,000) |   |
| Excess and Surplus Lines Insurance                                                                |            |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**           |            |            |   |   |
| Prior year claims and claims adjustment expense                                                   | $ (35,000,000)  | $ (29,000,000)  | $ (13,000,000) |   |

| X                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Asbestos and environmental reinsurance retention.
+ References
No definition available.
+ Details**Name:**cinf_AsbestosAndEnvironmentalReinsuranceRetention**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Increase in the recovery of liability for unpaid claims and claims adjustment expense, incurred claims, prior years.
+ References
No definition available.
+ Details**Name:**cinf_IncreaseInRecoveryOfLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to environmental claims related to mold.
+ References
No definition available.
+ Details**Name:**cinf_LiabilityForAsbestosAndEnvironmentalClaimsMold**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of reserves for asbestos and environmental claims to total loss and loss expense reserves.
+ References
No definition available.
+ Details**Name:**cinf_PercentageOfReservesForAsbestosAndEnvironmentalClaims**Namespace Prefix:**cinf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| 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
Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, net of estimated reinsurance recoveries.
+ 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) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437
+ Details**Name:**us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| 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                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of expense for claims incurred in prior reporting periods and related claims settlement costs.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                         |
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| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
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