| **Shareholders' Equity And Dividend Restrictions (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions**               | 12 Months Ended |         |        |
|-------------------------------------------------------------------------------------------------------------------------------------|-----------------|---------|--------|
||
| Declared cash dividends, per share                                                                                                  | $ 2.12          | $ 2.50  | $ 1.92 |   |   |
| Dividends declared by insurance subsidiary to parent                                                                                | $ 500           | $ 465   | $ 475  |   |   |
| Dividend payment without prior regulatory approval, maximum percent of policyholder surplus                                         | 10.00%          |    |   |   |   |
| Dividend payment without prior regulatory approval, percent of statutory net income for the prior calendar year                     | 100.00%         |    |   |   |   |
| Dividends might be paid during next year                                                                                            | $ 626           |    |   |   |   |
| Accumulated Other Comprehensive Income                                                                                              |            |    |   |   |   |
| Adjustment to reclassify certain tax effects from AOCI to retained earnings                                                         |            |    |   | $ 0 | $ 0 |
| Accumulated Other Comprehensive Income | New Accounting Pronouncement, Early Adoption, Effect | Accounting Standards Update 2018-02 |            |    |   |   |   |
| Adjustment to reclassify certain tax effects from AOCI to retained earnings                                                         | 0               | 492     | 0      |   |   |
| Retained Earnings | New Accounting Pronouncement, Early Adoption, Effect | Accounting Standards Update 2018-02                      |            |    |   |   |   |
| Adjustment to reclassify certain tax effects from AOCI to retained earnings                                                         | $ 0             | $ (492) | $ 0    |   |   |

| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Minimum percent of policyholder surplus that our insurance subsidiary can pay to the parent without prior regulatory approval.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum percentage of statutory net income that our insurance subsidiary can pay to the parent without prior regulatory approval.
+ References
No definition available.
+ Details**Name:**cinf_PercentageOfStatutoryNetIncomeFromPriorYear**Namespace Prefix:**cinf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate dividends declared during the period for each share of common stock outstanding.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of paid and unpaid cash, stock, and paid-in-kind (PIK) dividends declared, for example, but not limited to, common and preferred stock.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 405 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6957935&loc=d3e64057-112817
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of dividends available for payment after computation of statutory restrictions on capital surplus.
+ References
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| X                                                                                                                                                                               |
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| X                                                                                                                                                            |
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