| **Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions** | Sep. 30, 2019 | Dec. 31, 2018 |
|-------------------------------------------------------------------------------------|---------------|---------------|
| **Segment Reporting Information [Line Items]**                                      |          |          |
| Reinsurance receivables                                                             | $ 4,092       | $ 4,455       |
| Insurance receivables                                                               | 2,504         | 2,365         |
| Deferred acquisition costs                                                          | 668           | 633           |
| Goodwill                                                                            | 145           | 146           |
| Deferred non-insurance warranty acquisition expense                                 | 2,772         | 2,513         |
| **Insurance reserves**                                                              |          |          |
| Claim and claim adjustment expenses                                                 | 21,596        | 21,984        |
| Unearned premiums                                                                   | 4,608         | 4,183         |
| Future policy benefits                                                              | 12,305        | 10,597        |
| Deferred non-insurance warranty revenue                                             | 3,707         | 3,402         |
| Operating Segments | Specialty                                                      |          |          |
| **Segment Reporting Information [Line Items]**                                      |          |          |
| Reinsurance receivables                                                             | 599           | 649           |
| Insurance receivables                                                               | 992           | 947           |
| Deferred acquisition costs                                                          | 314           | 308           |
| Goodwill                                                                            | 117           | 117           |
| Deferred non-insurance warranty acquisition expense                                 | 2,772         | 2,513         |
| **Insurance reserves**                                                              |          |          |
| Claim and claim adjustment expenses                                                 | 5,285         | 5,465         |
| Unearned premiums                                                                   | 2,329         | 2,132         |
| Future policy benefits                                                              | 0             | 0             |
| Deferred non-insurance warranty revenue                                             | 3,707         | 3,402         |
| Operating Segments | Commercial                                                     |          |          |
| **Segment Reporting Information [Line Items]**                                      |          |          |
| Reinsurance receivables                                                             | 759           | 795           |
| Insurance receivables                                                               | 1,243         | 1,277         |
| Deferred acquisition costs                                                          | 264           | 230           |
| Goodwill                                                                            | 0             | 0             |
| Deferred non-insurance warranty acquisition expense                                 | 0             | 0             |
| **Insurance reserves**                                                              |          |          |
| Claim and claim adjustment expenses                                                 | 8,610         | 8,743         |
| Unearned premiums                                                                   | 1,679         | 1,454         |
| Future policy benefits                                                              | 0             | 0             |
| Deferred non-insurance warranty revenue                                             | 0             | 0             |
| Operating Segments | International                                                  |          |          |
| **Segment Reporting Information [Line Items]**                                      |          |          |
| Reinsurance receivables                                                             | 242           | 250           |
| Insurance receivables                                                               | 260           | 284           |
| Deferred acquisition costs                                                          | 90            | 95            |
| Goodwill                                                                            | 28            | 29            |
| Deferred non-insurance warranty acquisition expense                                 | 0             | 0             |
| **Insurance reserves**                                                              |          |          |
| Claim and claim adjustment expenses                                                 | 1,802         | 1,750         |
| Unearned premiums                                                                   | 475           | 475           |
| Future policy benefits                                                              | 0             | 0             |
| Deferred non-insurance warranty revenue                                             | 0             | 0             |
| Operating Segments | Life & Group                                               |          |          |
| **Segment Reporting Information [Line Items]**                                      |          |          |
| Reinsurance receivables                                                             | 394           | 414           |
| Insurance receivables                                                               | 8             | 9             |
| Deferred acquisition costs                                                          | 0             | 0             |
| Goodwill                                                                            | 0             | 0             |
| Deferred non-insurance warranty acquisition expense                                 | 0             | 0             |
| **Insurance reserves**                                                              |          |          |
| Claim and claim adjustment expenses                                                 | 3,707         | 3,601         |
| Unearned premiums                                                                   | 126           | 122           |
| Future policy benefits                                                              | 12,305        | 10,597        |
| Deferred non-insurance warranty revenue                                             | 0             | 0             |
| Operating Segments | Corporate & Other                                          |          |          |
| **Segment Reporting Information [Line Items]**                                      |          |          |
| Reinsurance receivables                                                             | 2,098         | 2,347         |
| Insurance receivables                                                               | 1             | (152)         |
| Deferred acquisition costs                                                          | 0             | 0             |
| Goodwill                                                                            | 0             | 0             |
| Deferred non-insurance warranty acquisition expense                                 | 0             | 0             |
| **Insurance reserves**                                                              |          |          |
| Claim and claim adjustment expenses                                                 | 2,192         | 2,425         |
| Unearned premiums                                                                   | 0             | 0             |
| Future policy benefits                                                              | 0             | 0             |
| Deferred non-insurance warranty revenue                                             | 0             | 0             |
| Eliminations                                                                        |          |          |
| **Segment Reporting Information [Line Items]**                                      |          |          |
| Reinsurance receivables                                                             | 0             | 0             |
| Insurance receivables                                                               | 0             | 0             |
| Deferred acquisition costs                                                          | 0             | 0             |
| Goodwill                                                                            | 0             | 0             |
| Deferred non-insurance warranty acquisition expense                                 | 0             | 0             |
| **Insurance reserves**                                                              |          |          |
| Claim and claim adjustment expenses                                                 | 0             | 0             |
| Unearned premiums                                                                   | (1)           | 0             |
| Future policy benefits                                                              | 0             | 0             |
| Deferred non-insurance warranty revenue                                             | $ 0           | $ 0           |

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| - Definition
Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time.
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| - Definition
Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred policy acquisition cost capitalized on contract remaining in force.
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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.
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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.
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, before effect of reinsurance, of present value of future benefit to be paid to or on behalf of policyholder and related expense less present value of future net premium receivable under insurance contract.
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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.
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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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Carrying amount of premiums written on insurance contracts that have not been earned as of the balance sheet date.
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