| **Deferred Policy Acquisition Costs (Deferred Policy Acquisition Costs And Asset Reconciliation Including The Amortized Deferred Policy Acquisition Costs) (Details) - USD ($) $ in Millions** | 12 Months Ended |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Deferred Costs [Roll Forward]**                                                                                                                                                              |            |   |   |
| Deferred policy acquisition costs asset at January 1                                                                                                                                           | $ 670           | $ 637 | $ 616 |
| Capitalized deferred policy acquisition costs                                                                                                                                                  | 993             | 948 | 889 |
| Amortized deferred policy acquisition costs                                                                                                                                                    | (946)           | (913) | (863) |
| Shadow deferred policy acquisition costs                                                                                                                                                       | 21              | (2) | (5) |
| Deferred policy acquisition costs asset at December 31                                                                                                                                         | 738             | 670 | 637 |
| Consolidated Property and Casualty Insurance Entity [Member]                                                                                                                                   |            |   |   |
| **Deferred Costs [Roll Forward]**                                                                                                                                                              |            |   |   |
| Deferred policy acquisition costs asset at January 1                                                                                                                                           | 438             | 408 | 388 |
| Capitalized deferred policy acquisition costs                                                                                                                                                  | 933             | 897 | 840 |
| Amortized deferred policy acquisition costs                                                                                                                                                    | (907)           | (867) | (820) |
| Deferred policy acquisition costs asset at December 31                                                                                                                                         | 464             | 438 | 408 |
| Life Insurance Product Line [Member]                                                                                                                                                           |            |   |   |
| **Deferred Costs [Roll Forward]**                                                                                                                                                              |            |   |   |
| Deferred policy acquisition costs asset at January 1                                                                                                                                           | 232             | 229 | 228 |
| Capitalized deferred policy acquisition costs                                                                                                                                                  | 60              | 51 | 49 |
| Amortized deferred policy acquisition costs                                                                                                                                                    | (39)            | (46) | (43) |
| Shadow deferred policy acquisition costs                                                                                                                                                       | 21              | (2) | (5) |
| Deferred policy acquisition costs asset at December 31                                                                                                                                         | $ 274           | $ 232 | $ 229 |

| X                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Deferred Costs
+ References
No definition available.
+ Details**Name:**cinf_DeferredCostsRollForward**Namespace Prefix:**cinf_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of amortization expense (reversal of expense) for deferred policy acquisition costs.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(7)(a)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 35 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=95462302&loc=d3e10157-158409Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16(Column I)) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column I)) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 45 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6480726&loc=d3e6691-158385Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35755530&loc=d3e11264-158415
+ Details**Name:**us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of other increase (decrease) to amortization expense of deferred policy acquisition cost (DAC).
+ References
No definition available.
+ Details**Name:**us-gaap_DeferredPolicyAcquisitionCostAmortizationExpenseOther**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net amount of deferred policy acquisition costs capitalized on contracts remaining in force as of the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 825 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6487548&loc=d3e32546-158582Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 55 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=68057816&loc=d3e11446-158418Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column B)) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6481648&loc=d3e11198-158413Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16(Column B)) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915Reference 6: 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)(7)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910
+ Details**Name:**us-gaap_DeferredPolicyAcquisitionCosts**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total amount of policy acquisition costs capitalized during the income statement period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35755530&loc=d3e11264-158415
+ Details**Name:**us-gaap_DeferredPolicyAcquisitionCostsAdditions**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=us-gaap_ConsolidatedPropertyAndCasualtyInsuranceEntityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=us-gaap_LifeInsuranceSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |