| **Summary of Investments Other Than Investments In Related Parties (Details)1 $ in Millions**                                  | Dec. 31, 2017 USD ($) |
|--------------------------------------------------------------------------------------------------------------------------------|-----------------------|
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | $ 13,511              |
| Balance sheet                                                                                                                  | 17,051                |
| Fixed Maturities                                                                                                               |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 10,314                |
| Fair value                                                                                                                     | 10,699                |
| Balance sheet                                                                                                                  | 10,699                |
| Equity Securities                                                                                                              |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 3,094                 |
| Fair value                                                                                                                     | 6,249                 |
| Balance sheet                                                                                                                  | 6,249                 |
| Other than Securities Investment                                                                                               |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 103                   |
| Balance sheet                                                                                                                  | 103                   |
| States, Municipalities and Political Subdivisions | Fixed Maturities                                                           |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 4,316                 |
| Fair value                                                                                                                     | 4,465                 |
| Balance sheet                                                                                                                  | 4,465                 |
| States, Municipalities and Political Subdivisions | Fixed Maturities | Cincinnati Financial Corporation                        |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 2                     |
| Fair value                                                                                                                     | 2                     |
| Balance sheet                                                                                                                  | 2                     |
| States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Insurance Company                        |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 3,310                 |
| Fair value                                                                                                                     | 3,421                 |
| Balance sheet                                                                                                                  | 3,421                 |
| States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Casualty Company                         |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 184                   |
| Fair value                                                                                                                     | 190                   |
| Balance sheet                                                                                                                  | 190                   |
| States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Indemnity Company                        |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 44                    |
| Fair value                                                                                                                     | 46                    |
| Balance sheet                                                                                                                  | 46                    |
| States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Life Insurance Company                   |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 306                   |
| Fair value                                                                                                                     | 320                   |
| Balance sheet                                                                                                                  | 320                   |
| States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Specialty Underwriters Insurance Company |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 469                   |
| Fair value                                                                                                                     | 485                   |
| Balance sheet                                                                                                                  | 485                   |
| States, Municipalities and Political Subdivisions | Fixed Maturities | CSU Producers Resources Inc.                            |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 1                     |
| Fair value                                                                                                                     | 1                     |
| Balance sheet                                                                                                                  | 1                     |
| United States Government | Fixed Maturities                                                                                    |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 31                    |
| Fair value                                                                                                                     | 31                    |
| Balance sheet                                                                                                                  | 31                    |
| United States Government | Fixed Maturities | The Cincinnati Insurance Company                                                 |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 28                    |
| Fair value                                                                                                                     | 28                    |
| Balance sheet                                                                                                                  | 28                    |
| United States Government | Fixed Maturities | The Cincinnati Casualty Company                                                  |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 2                     |
| Fair value                                                                                                                     | 2                     |
| Balance sheet                                                                                                                  | 2                     |
| United States Government | Fixed Maturities | The Cincinnati Indemnity Company                                                 |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 1                     |
| Fair value                                                                                                                     | 1                     |
| Balance sheet                                                                                                                  | 1                     |
| Government-Sponsored Enterprises | Fixed Maturities                                                                            |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 257                   |
| Fair value                                                                                                                     | 254                   |
| Balance sheet                                                                                                                  | 254                   |
| Government-Sponsored Enterprises | Fixed Maturities | The Cincinnati Life Insurance Company                                    |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 257                   |
| Fair value                                                                                                                     | 254                   |
| Balance sheet                                                                                                                  | 254                   |
| Foreign Government | Fixed Maturities                                                                                          |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 10                    |
| Fair value                                                                                                                     | 10                    |
| Balance sheet                                                                                                                  | 10                    |
| Foreign Government | Fixed Maturities | The Cincinnati Insurance Company                                                       |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 10                    |
| Fair value                                                                                                                     | 10                    |
| Balance sheet                                                                                                                  | 10                    |
| Common Stock | Equity Securities                                                                                               |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 2,918                 |
| Fair value                                                                                                                     | 6,039                 |
| Balance sheet                                                                                                                  | 6,039                 |
| Common Stock | Equity Securities | Cincinnati Financial Corporation                                                            |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 1,158                 |
| Fair value                                                                                                                     | 2,274                 |
| Balance sheet                                                                                                                  | 2,274                 |
| Common Stock | Equity Securities | The Cincinnati Insurance Company                                                            |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 1,615                 |
| Fair value                                                                                                                     | 3,479                 |
| Balance sheet                                                                                                                  | 3,479                 |
| Common Stock | Equity Securities | The Cincinnati Casualty Company                                                             |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 56                    |
| Fair value                                                                                                                     | 118                   |
| Balance sheet                                                                                                                  | 118                   |
| Common Stock | Equity Securities | The Cincinnati Indemnity Company                                                            |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 13                    |
| Fair value                                                                                                                     | 25                    |
| Balance sheet                                                                                                                  | 25                    |
| Common Stock | Equity Securities | The Cincinnati Specialty Underwriters Insurance Company                                     |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 61                    |
| Fair value                                                                                                                     | 120                   |
| Balance sheet                                                                                                                  | 120                   |
| Common Stock | Equity Securities | CSU Producers Resources Inc.                                                                |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 15                    |
| Fair value                                                                                                                     | 23                    |
| Balance sheet                                                                                                                  | 23                    |
| Nonredeemable Preferred Stock | Equity Securities                                                                              |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 176                   |
| Fair value                                                                                                                     | 210                   |
| Balance sheet                                                                                                                  | 210                   |
| Nonredeemable Preferred Stock | Equity Securities | Cincinnati Financial Corporation                                           |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 1                     |
| Fair value                                                                                                                     | 1                     |
| Balance sheet                                                                                                                  | 1                     |
| Nonredeemable Preferred Stock | Equity Securities | The Cincinnati Insurance Company                                           |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 170                   |
| Fair value                                                                                                                     | 200                   |
| Balance sheet                                                                                                                  | 200                   |
| Nonredeemable Preferred Stock | Equity Securities | The Cincinnati Life Insurance Company                                      |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 5                     |
| Fair value                                                                                                                     | 9                     |
| Balance sheet                                                                                                                  | 9                     |
| All Other Corporate Bonds | Fixed Maturities                                                                                   |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 5,700                 |
| Fair value                                                                                                                     | 5,939                 |
| Balance sheet                                                                                                                  | 5,939                 |
| All Other Corporate Bonds | Fixed Maturities | Cincinnati Financial Corporation                                                |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 33                    |
| Fair value                                                                                                                     | 35                    |
| Balance sheet                                                                                                                  | 35                    |
| All Other Corporate Bonds | Fixed Maturities | The Cincinnati Insurance Company                                                |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 2,644                 |
| Fair value                                                                                                                     | 2,747                 |
| Balance sheet                                                                                                                  | 2,747                 |
| All Other Corporate Bonds | Fixed Maturities | The Cincinnati Casualty Company                                                 |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 99                    |
| Fair value                                                                                                                     | 104                   |
| Balance sheet                                                                                                                  | 104                   |
| All Other Corporate Bonds | Fixed Maturities | The Cincinnati Indemnity Company                                                |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 28                    |
| Fair value                                                                                                                     | 29                    |
| Balance sheet                                                                                                                  | 29                    |
| All Other Corporate Bonds | Fixed Maturities | The Cincinnati Life Insurance Company                                           |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 2,758                 |
| Fair value                                                                                                                     | 2,880                 |
| Balance sheet                                                                                                                  | 2,880                 |
| All Other Corporate Bonds | Fixed Maturities | The Cincinnati Specialty Underwriters Insurance Company                         |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 137                   |
| Fair value                                                                                                                     | 143                   |
| Balance sheet                                                                                                                  | 143                   |
| All Other Corporate Bonds | Fixed Maturities | CSU Producers Resources Inc.                                                    |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 1                     |
| Fair value                                                                                                                     | 1                     |
| Balance sheet                                                                                                                  | 1                     |
| Policy Loans | Other than Securities Investment | The Cincinnati Life Insurance Company                                        |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 31                    |
| Fair value                                                                                                                     | 0                     |
| Balance sheet                                                                                                                  | 31                    |
| Limited Partnership | Other than Securities Investment | The Cincinnati Life Insurance Company                                 |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 6                     |
| Balance sheet                                                                                                                  | 6                     |
| Limited Partnership | Other than Securities Investment | Cincinnati Financial Corporation                                      |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 29                    |
| Fair value                                                                                                                     | 0                     |
| Balance sheet                                                                                                                  | 29                    |
| Real Estate [Member] | Other than Securities Investment | Cincinnati Financial Corporation                                     |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 6                     |
| Balance sheet                                                                                                                  | 6                     |
| Real Estate [Member] | Other than Securities Investment | The Cincinnati Life Insurance Company                                |                  |
| **Summary Of Investments Other Than Investments In Related Parties Reportable Data**                                           |                  |
| Cost or amortized cost                                                                                                         | 31                    |
| Balance sheet                                                                                                                  | $ 31                  |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the carrying (reported) amount of the entity's investments as of the balance sheet date. This amount may represent the individual investment's or group of similar investments' original cost, fair value, or such other amount at which the investment is shown in the entity's balance sheet.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 7 -Subparagraph (SX 210.12-15) -URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611379-123010
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the original cost of the entity's investments including investments in equity securities and, as to fixed maturities, original cost reduced by repayments and adjusted for amortization of premiums or accrual of discounts.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 7 -Subparagraph (SX 210.12-15) -URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611379-123010
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the reported amount of the entity's investments measured at fair value. Fair value is the price that would be received to sell an investment in an orderly transaction between market participants at the balance sheet [measurement] date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 7 -Subparagraph (SX 210.12-15) -URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611379-123010
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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|--------------------------------------------------------------------------------------------------------------------------------------------------|
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|---------------------------------------------------------------------------------------------------------------------------------------------------|
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|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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