| **Summary Of Significant Accounting Policies (Narrative) (Details)**                                                                | 12 Months Ended   |
|-------------------------------------------------------------------------------------------------------------------------------------|-------------------|
||
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Unrealized gains reclassified to retained earnings for ASU 2016-01, after tax                                                       | $ 2,503,000,000   |   |   |   |
| Excess tax benefit, amount                                                                                                          | 7,000,000         |   |   |   |
| Income Tax Examination, Penalties Expense                                                                                           | $ 0               |   |   |   |
| Number of operating subsidaries | subsidiary                                                                                        | 2                 |   |   |   |
| Number of independent insurance agencies | independent_insurance_agency                                                             | 1,702             |   |   |   |
| Number of reporting locations | reporting_locations                                                                                 | 2,256             |   |   |   |
| Number of states in which entity operates | state                                                                                   | 42                |   |   |   |
| Defined Benefit Plan Qualifying Age                                                                                                 |              |   |   | 40 years |
| Current fair value of the BOLI invested assets and cash exceeded the current fair value of the contract holders' account value      | $ 29,000,000      |   |   |   |
| Other invested assets                                                                                                               | 103,000,000       | $ 81,000,000 |   |   |
| Depreciation expense                                                                                                                | 28,000,000        | 30,000,000 | $ 36,000,000 |   |
| Land, building and equipment impairments                                                                                            | 0                 | 0 | 0 |   |
| Net cash used in financing activities                                                                                               | $ (614,000,000)   | (426,000,000) | (498,000,000) |   |
| Minimum                                                                                                                             |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Estimated useful lives                                                                                                              | 3 years           |   |   |   |
| Maximum                                                                                                                             |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Estimated useful lives                                                                                                              | 39 years 6 months |   |   |   |
| Life Policy Loans                                                                                                                   |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Other invested assets                                                                                                               | $ 31,000,000      | 31,000,000 |   |   |
| Venture Capital Funds                                                                                                               |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Other invested assets                                                                                                               | 35,000,000        | 23,000,000 |   |   |
| Real Estate Investment                                                                                                              |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Other invested assets                                                                                                               | $ 37,000,000      | $ 27,000,000 |   |   |
| 10 Largest States                                                                                                                   |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Generated percentage of total earned premiums                                                                                       | 59.70%            | 61.40% |   |   |
| Ohio, Largest State                                                                                                                 |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Generated percentage of total earned premiums                                                                                       | 16.20%            | 16.80% |   |   |
| Indiana, Illinois, Pennsylvania, Georgia, Michigan and North Carolina | Minimum                                                     |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Generated percentage of total earned premiums                                                                                       | 5.00%             |   |   |   |
| Indiana, Illinois, Pennsylvania, Georgia, Michigan and North Carolina | Maximum                                                     |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Generated percentage of total earned premiums                                                                                       | 7.00%             |   |   |   |
| Largest Single Agency Relationship                                                                                                  |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Generated percentage of total earned premiums                                                                                       | 0.80%             |   |   |   |
| All Other | Maximum                                                                                                                 |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Generated percentage of total earned premiums                                                                                       | 4.00%             |   |   |   |
| Stock Options                                                                                                                       |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Weighted-average remaining contractual life for options expected to vest                                                            | 10 years          |   |   |   |
| Property Casualty Insurance Segment                                                                                                 |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Number of operating subsidaries | subsidiary                                                                                        | 2                 |   |   |   |
| Stock Options                                                                                                                       |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Awards vesting period                                                                                                               | 3 years           |   |   |   |
| Restricted Stock Units (RSUs)                                                                                                       |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Awards vesting period                                                                                                               | 3 years           |   |   |   |
| Accounting Standards Update 2016-09                                                                                                 |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Net cash used in financing activities                                                                                               | $ 13,000,000      | $ 12,000,000 | 11,000,000 |   |
| Adjustment to reclassify certain tax effects from AOCI to retained earnings                                                         | 0                 |   |   |   |
| New Accounting Pronouncement, Early Adoption, Effect | Accumulated Other Comprehensive Income | Accounting Standards Update 2018-02 |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Adjustment to reclassify certain tax effects from AOCI to retained earnings                                                         | 492,000,000       | 0 | 0 |   |
| New Accounting Pronouncement, Early Adoption, Effect | Retained Earnings | Accounting Standards Update 2018-02                      |              |   |   |   |
| **Significant Accounting Policies**                                                                                                 |              |   |   |   |
| Adjustment to reclassify certain tax effects from AOCI to retained earnings                                                         | $ (492,000,000)   | $ 0 | $ 0 |   |

| X                                                                                                                                                                                                                                                                                                                     |
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Age at which participants could elect to remain in the defined benefit plan as of August 31, 2008.
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Independent Insurance Agencies and Brokerages
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Number of Locations
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Number of Subsidiaries
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Percentage of earned premiums.
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| - Definition
Unrealized gains on equity securities reclassified from accumulated other comprehensive income to retained earnings, after tax upon adoption of ASU 2016-01.
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Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.
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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.
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| - Definition
The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.
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| - Definition
The amount of estimated penalties recognized in the period arising from income tax examinations.
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| - Definition
Amount of cash inflow (outflow) from financing activities, including discontinued operations. Financing activity cash flows include obtaining resources from owners and providing them with a return on, and a return of, their investment; borrowing money and repaying amounts borrowed, or settling the obligation; and obtaining and paying for other resources obtained from creditors on long-term credit.
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| - Definition
The number of states the entity operates in as of the balance sheet date.
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Amount of investments classified as other.
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| - Definition
Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.
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| - Definition
Fair value amount of the specified investment category of assets maintained in separate accounts as of the balance sheet date.
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| - Definition
Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
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| - Definition
Weighted average remaining contractual term of exercisable stock options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
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