| **Statutory Accounting Information (Schedule Of Statutory Net Income And Statutory Surplus (Details) (USD $)In Millions, unless otherwise specified** | 12 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
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| Cincinnati Insurance Company [Member]                                                                                                                 |            |   |   |
| **Statutory Accounting Practices [Line Items]**                                                                                                       |            |   |   |
| SAP Net Income (Loss)                                                                                                                                 | $ 418           | $ 334 | $ 120 |
| Capital and Surplus                                                                                                                                   | 4,326           | 3,914 |   |
| Cincinnati Casualty Company [Member]                                                                                                                  |            |   |   |
| **Statutory Accounting Practices [Line Items]**                                                                                                       |            |   |   |
| SAP Net Income (Loss)                                                                                                                                 | 10              | 10 | 15 |
| Capital and Surplus                                                                                                                                   | 317             | 293 |   |
| Cincinnati Indemnity Company [Member]                                                                                                                 |            |   |   |
| **Statutory Accounting Practices [Line Items]**                                                                                                       |            |   |   |
| SAP Net Income (Loss)                                                                                                                                 | 2               | 2 | 2 |
| Capital and Surplus                                                                                                                                   | 82              | 76 |   |
| Cincinnati Specialty Underwriters Insurance Company [Member]                                                                                          |            |   |   |
| **Statutory Accounting Practices [Line Items]**                                                                                                       |            |   |   |
| SAP Net Income (Loss)                                                                                                                                 | 18              | 6 | 11 |
| Capital and Surplus                                                                                                                                   | 228             | 199 |   |
| Cincinnati Life Insurance Company [Member]                                                                                                            |            |   |   |
| **Statutory Accounting Practices [Line Items]**                                                                                                       |            |   |   |
| SAP Net Income (Loss)                                                                                                                                 | (20)            | 5 | (13) |
| Capital and Surplus                                                                                                                                   | $ 247           | $ 276 |   |

| X                                                                                                                                                                         |
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| - Details**Name:**us-gaap_StatutoryAccountingPracticesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.23(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24708-158529Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 24 -Subparagraph c -Article 7
+ Details**Name:**us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of net income for the period determined using accounting principles prescribed or permitted by insurance regulators.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.23(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 24 -Subparagraph c -Article 7
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