| **Income Taxes Reconciliation of Cincinnati Global Valuation Allowance (Details) - USD ($) $ in Millions** | 12 Months Ended |
|------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Valuation Allowance**                                                                                    |            |   |   |
| Valuation allowance at January 1                                                                           | $ 56            |   |   |
| Valuation allowance at December 31                                                                         | 53              | $ 56 |   |
| Cincinnati Global                                                                                          |            |   |   |
| **Valuation Allowance**                                                                                    |            |   |   |
| Valuation allowance at January 1                                                                           | 56              | 41 | $ 0 |
| Acquisition accounting amount                                                                              | 0               | 0 | 55 |
| Current year operations                                                                                    | (3)             | 15 | (14) |
| Valuation allowance at December 31                                                                         | $ 53            | $ 56 | $ 41 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121826272&loc=d3e32537-109319
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the valuation allowance recorded in a business combination against deductible temporary differences for which related tax benefits will be recorded as a reduction of the acquired entity's income tax expense (after such benefits are first being applied to reduce goodwill and then other noncurrent intangible assets to zero).
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 740 -Section 25 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=123403335&loc=d3e9901-128506
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=121826272&loc=d3e32537-109319
+ Details**Name:**us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| 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_ValuationAllowanceLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_BusinessAcquisitionAxis=cinf_CincinnatiGlobalUnderwritingLtd.Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |