| **Supplementary Insurance Information (Details) - USD ($) $ in Millions** | 12 Months Ended |
|---------------------------------------------------------------------------|-----------------|
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| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Deferred policy acquisition costs                                         | $ 738           | $ 670 | $ 637 |
| Gross future policy benefits, losses, claims and expense losses           | 8,448           | 7,972 | 7,728 |
| Gross unearned premium                                                    | 2,516           | 2,404 | 2,307 |
| Other policy claims and benefits payable                                  | 38              | 30 | 28 |
| Earned premiums                                                           | 5,170           | 4,954 | 4,710 |
| Investment income, net of expenses                                        | 554             | 547 | 539 |
| Benefits, claims losses and settlement expense                            | 3,490           | 3,390 | 3,107 |
| Amortization of deferred policy acquisition costs                         | 946             | 913 | 863 |
| Underwriting, acquisition and insurance expenses                          | 651             | 633 | 602 |
| Net written premiums                                                      | 5,033           | 4,843 | 4,582 |
| Commercial Insurance                                                      |            |   |   |
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Deferred policy acquisition costs                                         | 291             | 284 | 271 |
| Gross future policy benefits, losses, claims and expense losses           | 4,466           | 4,236 | 4,179 |
| Gross unearned premium                                                    | 1,576           | 1,548 | 1,510 |
| Earned premiums                                                           | 3,218           | 3,165 | 3,089 |
| Benefits, claims losses and settlement expense                            | 2,049           | 2,042 | 1,928 |
| Amortization of deferred policy acquisition costs                         | 608             | 590 | 570 |
| Underwriting, acquisition and insurance expenses                          | 415             | 419 | 412 |
| Net written premiums                                                      | 3,245           | 3,202 | 3,122 |
| Personal Insurance                                                        |            |   |   |
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Deferred policy acquisition costs                                         | 126             | 121 | 110 |
| Gross future policy benefits, losses, claims and expense losses           | 679             | 587 | 569 |
| Gross unearned premium                                                    | 725             | 683 | 629 |
| Earned premiums                                                           | 1,336           | 1,241 | 1,161 |
| Benefits, claims losses and settlement expense                            | 972             | 918 | 840 |
| Amortization of deferred policy acquisition costs                         | 242             | 225 | 209 |
| Underwriting, acquisition and insurance expenses                          | 147             | 135 | 128 |
| Net written premiums                                                      | 1,378           | 1,294 | 1,198 |
| Excess and Surplus Lines Insurance                                        |            |   |   |
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Deferred policy acquisition costs                                         | 21              | 17 | 16 |
| Gross future policy benefits, losses, claims and expense losses           | 298             | 264 | 241 |
| Gross unearned premium                                                    | 123             | 105 | 93 |
| Earned premiums                                                           | 234             | 209 | 183 |
| Benefits, claims losses and settlement expense                            | 104             | 86 | 68 |
| Amortization of deferred policy acquisition costs                         | 39              | 35 | 31 |
| Underwriting, acquisition and insurance expenses                          | 29              | 28 | 23 |
| Net written premiums                                                      | 249             | 219 | 189 |
| Reinsurance assumed and other non segment [Member]                        |            |   |   |
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Deferred policy acquisition costs                                         | 26              | 16 | 11 |
| Gross future policy benefits, losses, claims and expense losses           | 203             | 132 | 46 |
| Gross unearned premium                                                    | 91              | 67 | 74 |
| Earned premiums                                                           | 132             | 107 | 49 |
| Benefits, claims losses and settlement expense                            | 98              | 92 | 25 |
| Amortization of deferred policy acquisition costs                         | 18              | 17 | 10 |
| Underwriting, acquisition and insurance expenses                          | 24              | 18 | 6 |
| Net written premiums                                                      | 158             | 125 | 71 |
| Property, Liability and Casualty Insurance Product Line [Member]          |            |   |   |
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Deferred policy acquisition costs                                         | 464             | 438 | 408 |
| Gross future policy benefits, losses, claims and expense losses           | 5,646           | 5,219 | 5,035 |
| Gross unearned premium                                                    | 2,515           | 2,403 | 2,306 |
| Earned premiums                                                           | 4,920           | 4,722 | 4,482 |
| Investment income, net of expenses                                        | 401             | 392 | 384 |
| Benefits, claims losses and settlement expense                            | 3,223           | 3,138 | 2,861 |
| Amortization of deferred policy acquisition costs                         | 907             | 867 | 820 |
| Underwriting, acquisition and insurance expenses                          | 615             | 600 | 569 |
| Net written premiums                                                      | 5,030           | 4,840 | 4,580 |
| Accident Health Insurance Segment [Member]                                |            |   |   |
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Net written premiums                                                      | 3               | 3 | 2 |
| Life Insurance Segment                                                    |            |   |   |
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Deferred policy acquisition costs                                         | 274             | 232 | 229 |
| Gross future policy benefits, losses, claims and expense losses           | 2,802           | 2,753 | 2,693 |
| Gross unearned premium                                                    | 1               | 1 | 1 |
| Other policy claims and benefits payable                                  | 38              | 30 | 28 |
| Earned premiums                                                           | 250             | 232 | 228 |
| Investment income, net of expenses                                        | 153             | 155 | 155 |
| Benefits, claims losses and settlement expense                            | 267             | 252 | 246 |
| Amortization of deferred policy acquisition costs                         | 39              | 46 | 43 |
| Underwriting, acquisition and insurance expenses                          | $ 36            | $ 33 | $ 33 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of amortization of deferred policy acquisition costs recognized.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of insurance benefits, claims, losses and settlement expenses incurred.
+ References
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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.
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No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net amount of deferred policy acquisition costs capitalized on contracts remaining in force on the balance sheet date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of reserves for future policy claims payable and loss expenses to be incurred.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of net investment income earned.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of other operating expense incurred.
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| - Definition
Amount of claims and benefits payable classified as other.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
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| - Definition
Amount of premium revenue earned.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
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| - Definition
Amount of actual written premiums, including, as applicable, premiums from reinsurance assumed and net of premiums on reinsurance ceded.
+ References
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| - Definition
Amount of premiums not yet earned.
+ References
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