| **Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions** | Dec. 31, 2017 | Dec. 31, 2016 |
|-------------------------------------------------------------------------------------|---------------|---------------|
| **Segment Reporting Information [Line Items]**                                      |          |          |
| Reinsurance receivables                                                             | $ 4,290       | $ 4,453       |
| Insurance receivables                                                               | 2,336         | 2,255         |
| Deferred acquisition costs                                                          | 634           | 600           |
| Goodwill                                                                            | 148           | 145           |
| **Insurance reserves**                                                              |          |          |
| Claim and claim adjustment expenses                                                 | 22,004        | 22,343        |
| Unearned premiums                                                                   | 4,029         | 3,762         |
| Future policy benefits                                                              | 11,179        | 10,326        |
| Operating Segments | Specialty                                                      |          |          |
| **Segment Reporting Information [Line Items]**                                      |          |          |
| Reinsurance receivables                                                             | 678           | 760           |
| Insurance receivables                                                               | 984           | 982           |
| Deferred acquisition costs                                                          | 319           | 310           |
| Goodwill                                                                            | 117           | 117           |
| **Insurance reserves**                                                              |          |          |
| Claim and claim adjustment expenses                                                 | 5,848         | 6,149         |
| Unearned premiums                                                                   | 2,035         | 1,911         |
| Future policy benefits                                                              | 0             | 0             |
| Operating Segments | Commercial                                                     |          |          |
| **Segment Reporting Information [Line Items]**                                      |          |          |
| Reinsurance receivables                                                             | 647           | 621           |
| Insurance receivables                                                               | 1,088         | 1,021         |
| Deferred acquisition costs                                                          | 222           | 214           |
| Goodwill                                                                            | 0             | 0             |
| **Insurance reserves**                                                              |          |          |
| Claim and claim adjustment expenses                                                 | 8,585         | 8,894         |
| Unearned premiums                                                                   | 1,394         | 1,323         |
| Future policy benefits                                                              | 0             | 0             |
| Operating Segments | International                                                  |          |          |
| **Segment Reporting Information [Line Items]**                                      |          |          |
| Reinsurance receivables                                                             | 212           | 131           |
| Insurance receivables                                                               | 254           | 233           |
| Deferred acquisition costs                                                          | 93            | 76            |
| Goodwill                                                                            | 31            | 28            |
| **Insurance reserves**                                                              |          |          |
| Claim and claim adjustment expenses                                                 | 1,636         | 1,328         |
| Unearned premiums                                                                   | 472           | 396           |
| Future policy benefits                                                              | 0             | 0             |
| Operating Segments | Life & Group                                               |          |          |
| **Segment Reporting Information [Line Items]**                                      |          |          |
| Reinsurance receivables                                                             | 438           | 462           |
| Insurance receivables                                                               | 8             | 17            |
| Deferred acquisition costs                                                          | 0             | 0             |
| Goodwill                                                                            | 0             | 0             |
| **Insurance reserves**                                                              |          |          |
| Claim and claim adjustment expenses                                                 | 3,499         | 3,358         |
| Unearned premiums                                                                   | 128           | 132           |
| Future policy benefits                                                              | 11,179        | 10,326        |
| Operating Segments | Corporate & Other                                          |          |          |
| **Segment Reporting Information [Line Items]**                                      |          |          |
| Reinsurance receivables                                                             | 2,315         | 2,479         |
| Insurance receivables                                                               | 2             | 2             |
| Deferred acquisition costs                                                          | 0             | 0             |
| Goodwill                                                                            | 0             | 0             |
| **Insurance reserves**                                                              |          |          |
| Claim and claim adjustment expenses                                                 | 2,436         | 2,614         |
| Unearned premiums                                                                   | 0             | 0             |
| Future policy benefits                                                              | 0             | 0             |
| Eliminations                                                                        |          |          |
| **Segment Reporting Information [Line Items]**                                      |          |          |
| Reinsurance receivables                                                             | 0             | 0             |
| Insurance receivables                                                               | 0             | 0             |
| Deferred acquisition costs                                                          | 0             | 0             |
| Goodwill                                                                            | 0             | 0             |
| **Insurance reserves**                                                              |          |          |
| Claim and claim adjustment expenses                                                 | 0             | 0             |
| Unearned premiums                                                                   | 0             | 0             |
| Future policy benefits                                                              | $ 0           | $ 0           |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net amount of deferred policy acquisition costs capitalized on contracts remaining in force as of the balance sheet date.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of accrued obligation to policyholders that relates to insured events for long-duration contracts.
+ References
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| - Definition
Gross amount, as of the balance sheet date, due the entity from (a) agents and insureds, (b) uncollected premiums and (c) others, before deducting the allowance for doubtful accounts.
+ References
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| - Definition
Amount before valuation allowance recoverable under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.
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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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| - Definition
Carrying amount of premiums written on insurance contracts that have not been earned as of the balance sheet date.
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