| **Statutory Accounting Information (Schedule Of Statutory Net Income And Statutory Surplus (Details) - USD ($) $ in Millions** | 12 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| The Cincinnati Insurance Company                                                                                               |            |   |   |
| **Statutory Accounting Practices**                                                                                             |            |   |   |
| SAP net income (loss)                                                                                                          | $ 434           | $ 534 | $ 436 |
| Capital and surplus                                                                                                            | 4,686           | 4,412 |   |
| The Cincinnati Casualty Company                                                                                                |            |   |   |
| **Statutory Accounting Practices**                                                                                             |            |   |   |
| SAP net income (loss)                                                                                                          | 11              | 12 | 12 |
| Capital and surplus                                                                                                            | 360             | 337 |   |
| The Cincinnati Indemnity Company                                                                                               |            |   |   |
| **Statutory Accounting Practices**                                                                                             |            |   |   |
| SAP net income (loss)                                                                                                          | 4               | 3 | 3 |
| Capital and surplus                                                                                                            | 93              | 88 |   |
| The Cincinnati Specialty Underwriters Insurance Company                                                                        |            |   |   |
| **Statutory Accounting Practices**                                                                                             |            |   |   |
| SAP net income (loss)                                                                                                          | 57              | 49 | 32 |
| Capital and surplus                                                                                                            | 372             | 306 |   |
| The Cincinnati Life Insurance Company                                                                                          |            |   |   |
| **Statutory Accounting Practices**                                                                                             |            |   |   |
| SAP net income (loss)                                                                                                          | 2               | (11) | $ (19) |
| Capital and surplus                                                                                                            | $ 200           | $ 208 |   |

| 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.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
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| - 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(a)(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=d3e24708-158529
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| 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(a)(23)(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910
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| X                                                                                                                                                 |
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| X                                                                                                                                                |
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| X                                                                                                                                                 |
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| X                                                                                                                                                                      |
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| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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