| **Segment Information**          | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
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| **Segment Reporting [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
| Segment Information              | Segment InformationWe operate primarily in two industries, property casualty insurance and life insurance. We regularly review our reporting segments to make decisions about allocating resources and assessing performance. Our reporting segments are:•Commercial lines insurance•Personal lines insurance•Excess and surplus lines insurance•Life insurance•InvestmentsWe report as Other the noninvestment operations of the parent company and its noninsurer subsidiary, CFC Investment Company and Cincinnati Re, our reinsurance assumed operations. See our 2015 Annual Report on Form 10-K, Item 8, Note 18, Segment Information, Page 155, for a description of revenue, income or loss before income taxes and identifiable assets for each of the five segments.Segment information is summarized in the following table: (Dollars in millions)Three months ended March 31, 2016 2015Revenues:   Commercial lines insurance   Commercial casualty$257 $244Commercial property214 196Commercial auto144 136Workers' compensation89 93Other commercial 56 64Commercial lines insurance premiums760 733Fee revenues1 1Total commercial lines insurance761 734    Personal lines insurance   Personal auto131 123Homeowner119 114Other personal33 31Personal lines insurance premiums283 268Fee revenues1 1Total personal lines insurance284 269    Excess and surplus lines insurance43 40    Life insurance premiums58 53Separate account investment management fees1 1Total life insurance59 54    Investments       Investment income, net of expenses145 139    Realized investment gains, net61 47Total investment revenue206 186    Other11 2Total revenues$1,364 $1,285    Income (loss) before income taxes:   Insurance underwriting results   Commercial lines insurance$50 $26Personal lines insurance28 (3)Excess and surplus lines insurance17 4Life insurance(1) (3)Investments184 165Other(13) (15)Total income before income taxes$265 $174Identifiable assets: March 31, 2016 December 31, 2015Property casualty insurance $2,699 $2,717Life insurance 1,372 1,325Investments 14,978 14,485Other 398 361Total $19,447 $18,888      |

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| - Definition
The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.
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