| **Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions**          | 3 Months Ended | 6 Months Ended |         |         |         |         |
|---------------------------------------------------------------------------------------|----------------|----------------|---------|---------|---------|---------|
||
| **Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]**              |           |           |    |    |    |    |   |   |
| Accumulated other comprehensive income, net of tax, beginning of period               | $ 1,514        |           | $ 1,514 |    |    | $ 1,744 |   |   |
| Accumulated other comprehensive income, net of tax, end of period                     | 1,514          |           | 1,514   |    |    |    |   |   |
| Accumulated Net Unrealized Investment Gain (Loss)                                     |           |           |    |    |    |    |   |   |
| **Accumulated Other Comprehensive Income Loss Before Tax [Abstract]**                 |           |           |    |    |    |    |   |   |
| Accumulated other comprehensive income, before tax, beginning of period               | 2,676          | $ 2,452        | 2,719   | $ 2,335 |    |    |   |   |
| Other comprehensive income before reclassification, before tax                        | (260)          | 252            | (257)   | 387     |    |    |   |   |
| Reclassification adjustment, before tax                                               | (59)           | (17)           | (105)   | (35)    |    |    |   |   |
| Effect on other comprehensive income, before tax                                      | (319)          | 235            | (362)   | 352     |    |    |   |   |
| Accumulated other comprehensive income, before tax, end of period                     | 2,357          | 2,687          | 2,357   | 2,687   |    |    |   |   |
| **Accumulated Other Comprehensive Income (Loss), Tax [Abstract]**                     |           |           |    |    |    |    |   |   |
| Accumulated other comprehensive income, tax, beginning of period                      | 927            | 849            | 942     | 808     |    |    |   |   |
| Other comprehensive income before reclassification, tax                               | (91)           | 87             | (90)    | 135     |    |    |   |   |
| Reclassification adjustment, tax                                                      | (21)           | (5)            | (37)    | (12)    |    |    |   |   |
| Effect on other comprehensive income, tax                                             | (112)          | 82             | (127)   | 123     |    |    |   |   |
| Accumulated other comprehensive income, tax, end of period                            | 815            | 931            | 815     | 931     |    |    |   |   |
| **Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]**              |           |           |    |    |    |    |   |   |
| Accumulated other comprehensive income, net of tax, beginning of period               | 1,542          | 1,756          | 1,542   | 1,756   | $ 1,749 | 1,777   | $ 1,603 | $ 1,527 |
| Other comprehensive income before reclassification, net of tax                        | (169)          | 165            | (167)   | 252     |    |    |   |   |
| Reclassification adjustment, net of tax                                               | (38)           | (12)           | (68)    | (23)    |    |    |   |   |
| Effect on other comprehensive income, net of tax                                      | (207)          | 153            | (235)   | 229     |    |    |   |   |
| Accumulated other comprehensive income, net of tax, end of period                     | 1,542          | 1,756          | 1,542   | 1,756   |    |    |   |   |
| Accumulated Defined Benefit Plans Adjustment                                          |           |           |    |    |    |    |   |   |
| **Accumulated Other Comprehensive Income Loss Before Tax [Abstract]**                 |           |           |    |    |    |    |   |   |
| Accumulated other comprehensive income, before tax, beginning of period               | (35)           | (19)           | (36)    | (18)    |    |    |   |   |
| Other comprehensive income before reclassification, before tax                        | 0              | 0              | 0       | 0       |    |    |   |   |
| Reclassification adjustment, before tax                                               | 2              | 0              | 3       | (1)     |    |    |   |   |
| Effect on other comprehensive income, before tax                                      | 2              | 0              | 3       | (1)     |    |    |   |   |
| Accumulated other comprehensive income, before tax, end of period                     | (33)           | (19)           | (33)    | (19)    |    |    |   |   |
| **Accumulated Other Comprehensive Income (Loss), Tax [Abstract]**                     |           |           |    |    |    |    |   |   |
| Accumulated other comprehensive income, tax, beginning of period                      | (12)           | (6)            | (12)    | (6)     |    |    |   |   |
| Other comprehensive income before reclassification, tax                               | 0              | 0              | 0       | 0       |    |    |   |   |
| Reclassification adjustment, tax                                                      | 1              | 0              | 1       | 0       |    |    |   |   |
| Effect on other comprehensive income, tax                                             | 1              | 0              | 1       | 0       |    |    |   |   |
| Accumulated other comprehensive income, tax, end of period                            | (11)           | (6)            | (11)    | (6)     |    |    |   |   |
| **Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]**              |           |           |    |    |    |    |   |   |
| Accumulated other comprehensive income, net of tax, beginning of period               | (22)           | (13)           | (22)    | (13)    | (23)    | (24)    | (13) | (12) |
| Other comprehensive income before reclassification, net of tax                        | 0              | 0              | 0       | 0       |    |    |   |   |
| Reclassification adjustment, net of tax                                               | 1              | 0              | 2       | (1)     |    |    |   |   |
| Effect on other comprehensive income, net of tax                                      | 1              | 0              | 2       | (1)     |    |    |   |   |
| Accumulated other comprehensive income, net of tax, end of period                     | (22)           | (13)           | (22)    | (13)    |    |    |   |   |
| Accumulated Net Unrealized Gain (Loss) on Deferred Costs, Reserves And Other          |           |           |    |    |    |    |   |   |
| **Accumulated Other Comprehensive Income Loss Before Tax [Abstract]**                 |           |           |    |    |    |    |   |   |
| Accumulated other comprehensive income, before tax, beginning of period               | (14)           | (17)           | (12)    | (16)    |    |    |   |   |
| Other comprehensive income before reclassification, before tax                        | 7              | (2)            | 6       | 1       |    |    |   |   |
| Reclassification adjustment, before tax                                               | (1)            | 3              | (2)     | (1)     |    |    |   |   |
| Effect on other comprehensive income, before tax                                      | 6              | 1              | 4       | 0       |    |    |   |   |
| Accumulated other comprehensive income, before tax, end of period                     | (8)            | (16)           | (8)     | (16)    |    |    |   |   |
| **Accumulated Other Comprehensive Income (Loss), Tax [Abstract]**                     |           |           |    |    |    |    |   |   |
| Accumulated other comprehensive income, tax, beginning of period                      | (4)            | (5)            | (3)     | (5)     |    |    |   |   |
| Other comprehensive income before reclassification, tax                               | 3              | 0              | 2       | 1       |    |    |   |   |
| Reclassification adjustment, tax                                                      | (1)            | 0              | (1)     | (1)     |    |    |   |   |
| Effect on other comprehensive income, tax                                             | 2              | 0              | 1       | 0       |    |    |   |   |
| Accumulated other comprehensive income, tax, end of period                            | (2)            | (5)            | (2)     | (5)     |    |    |   |   |
| **Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]**              |           |           |    |    |    |    |   |   |
| Accumulated other comprehensive income, net of tax, beginning of period               | (6)            | (11)           | (6)     | (11)    | (10)    | (9)     | (12) | (11) |
| Other comprehensive income before reclassification, net of tax                        | 4              | (2)            | 4       | 0       |    |    |   |   |
| Reclassification adjustment, net of tax                                               | 0              | 3              | (1)     | 0       |    |    |   |   |
| Effect on other comprehensive income, net of tax                                      | 4              | 1              | 3       | 0       |    |    |   |   |
| Accumulated other comprehensive income, net of tax, end of period                     | (6)            | (11)           | (6)     | (11)    |    |    |   |   |
| Accumulated Other Comprehensive Income (Loss)                                         |           |           |    |    |    |    |   |   |
| **Accumulated Other Comprehensive Income Loss Before Tax [Abstract]**                 |           |           |    |    |    |    |   |   |
| Accumulated other comprehensive income, before tax, beginning of period               | 2,627          | 2,416          | 2,671   | 2,301   |    |    |   |   |
| Change in unrealized gains, net, on investments available for sale, before tax        | (319)          | 235            | (362)   | 352     |    |    |   |   |
| Change in pension obligations, before tax                                             | 2              | 0              | 3       | (1)     |    |    |   |   |
| Change in life deferred acquisition costs, life policy reserves and other, before tax | 6              | 1              | 4       | 0       |    |    |   |   |
| Effect on other comprehensive income, before tax                                      | (311)          | 236            | (355)   | 351     |    |    |   |   |
| Accumulated other comprehensive income, before tax, end of period                     | 2,316          | 2,652          | 2,316   | 2,652   |    |    |   |   |
| **Accumulated Other Comprehensive Income (Loss), Tax [Abstract]**                     |           |           |    |    |    |    |   |   |
| Accumulated other comprehensive income, tax, beginning of period                      | 911            | 838            | 927     | 797     |    |    |   |   |
| Change in unrealized gains, net, on investments available for sale, tax               | (112)          | 82             | (127)   | 123     |    |    |   |   |
| Change in pension obligations, tax                                                    | 1              | 0              | 1       | 0       |    |    |   |   |
| Change in life deferred acquisition costs, life policy reserves and other, tax        | 2              | 0              | 1       | 0       |    |    |   |   |
| Effect on other comprehensive income, tax                                             | (109)          | 82             | (125)   | 123     |    |    |   |   |
| Accumulated other comprehensive income, tax, end of period                            | 802            | 920            | 802     | 920     |    |    |   |   |
| **Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]**              |           |           |    |    |    |    |   |   |
| Accumulated other comprehensive income, net of tax, beginning of period               | 1,514          | 1,732          | 1,514   | 1,732   | $ 1,716 | $ 1,744 | $ 1,578 | $ 1,504 |
| Change in unrealized gains, net, on investments available for sale, net of tax        | (207)          | 153            | (235)   | 229     |    |    |   |   |
| Change in pension obligations, net of tax                                             | 1              | 0              | 2       | (1)     |    |    |   |   |
| Change in life deferred acquisition costs, life policy reserves and other, net of tax | 4              | 1              | 3       | 0       |    |    |   |   |
| Effect on other comprehensive income, net of tax                                      | (202)          | 154            | (230)   | 228     |    |    |   |   |
| Accumulated other comprehensive income, net of tax, end of period                     | $ 1,514        | $ 1,732        | $ 1,514 | $ 1,732 |    |    |   |   |

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Increase (Decrease) in Other Comprehensive Income (Loss), before Tax
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Other Comprehensive Income (Loss), Available-for-sale Securities and Other Assets Adjustment, before Tax
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Other Comprehensive Income (Loss), Available-for-sale Securities and Other Assets Adjustment, Net of Tax
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Other Comprehensive Income (Loss), Before Reclassifications, Tax
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Other Comprehensive Income (Loss), Deferred Acquisition Costs, Life Policy Reserves and Other Adjustment, before Tax
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Other Comprehensive Income (Loss), Deferred Acquisition Costs, Life Policy Reserves and Other Adjustment, Net of Tax
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Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax
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| - Definition
Amount before tax of increase (decrease) in accumulated in equity from transactions and other events and circumstances from non-owner sources. Excludes net income (loss), and accumulated changes in equity from transactions resulting from investments by owners (distributions to owners).
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| - Definition
Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.
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Amount before tax and reclassification adjustments of other comprehensive income (loss).
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Amount after tax, before reclassification adjustments of other comprehensive income (loss).
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| - Definition
Amount before tax of other comprehensive income (loss) attributable to parent entity.
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Amount after tax of other comprehensive income (loss) attributable to parent entity.
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Amount before tax, after reclassification adjustments, of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans.
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Amount after tax and reclassification adjustments, of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans.
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| - Definition
Amount of tax (expense) benefit, after reclassification adjustments, of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans.
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| - Definition
Amount of tax expense (benefit) allocated to other comprehensive income (loss) attributable to parent entity.
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Amount before tax of reclassification adjustments of other comprehensive income (loss).
+ References
No definition available.
+ Details**Name:**us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
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| - Definition
Amount after tax of reclassification adjustments of other comprehensive income (loss).
+ References
No definition available.
+ Details**Name:**us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                      |
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| - Details**Name:**us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                    |
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| - Details**Name:**us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                        |
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| - Details**Name:**us-gaap_StatementEquityComponentsAxis=cinf_AccumulatedNetUnrealizedGainLossOnDeferredCostsReservesAndOtherMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                               |
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| - Details**Name:**us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedOtherComprehensiveIncomeMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |