| **Benefit Plans (Fair Value of Plan Assets Measured on a Recurring Basis) (Details) - USD ($) $ in Millions** | Dec. 31, 2017 | Dec. 31, 2016 |
|---------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Other assets                                                                                                  | $ 2,261       | $ 2,193       |
| Corporate and other bonds                                                                                     |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 532           | 510           |
| Corporate and other bonds | Level 1                                                                           |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 0             | 0             |
| Corporate and other bonds | Level 2                                                                           |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 522           | 500           |
| Corporate and other bonds | Level 3                                                                           |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 10            | 10            |
| States, municipalities and political subdivisions                                                             |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 62            | 63            |
| States, municipalities and political subdivisions | Level 1                                                   |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 0             | 0             |
| States, municipalities and political subdivisions | Level 2                                                   |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 62            | 63            |
| States, municipalities and political subdivisions | Level 3                                                   |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 0             | 0             |
| Total asset-backed                                                                                            |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 180           | 179           |
| Total asset-backed | Level 1                                                                                  |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 0             | 0             |
| Total asset-backed | Level 2                                                                                  |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 180           | 179           |
| Total asset-backed | Level 3                                                                                  |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 0             | 0             |
| Total fixed maturity securities trading                                                                       |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 774           | 752           |
| Total fixed maturity securities trading | Level 1                                                             |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 0             | 0             |
| Total fixed maturity securities trading | Level 2                                                             |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 764           | 742           |
| Total fixed maturity securities trading | Level 3                                                             |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 10            | 10            |
| Equity securities                                                                                             |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 527           | 468           |
| Equity securities | Level 1                                                                                   |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 405           | 363           |
| Equity securities | Level 2                                                                                   |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 122           | 105           |
| Equity securities | Level 3                                                                                   |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 0             | 0             |
| Short term investments                                                                                        |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 34            | 46            |
| Short term investments | Level 1                                                                              |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 23            | 11            |
| Short term investments | Level 2                                                                              |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 11            | 35            |
| Short term investments | Level 3                                                                              |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 0             | 0             |
| Other assets                                                                                                  |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 9             | 37            |
| Other assets | Level 1                                                                                        |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 0             | 0             |
| Other assets | Level 2                                                                                        |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 9             | 37            |
| Other assets | Level 3                                                                                        |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 0             | 0             |
| Cash                                                                                                          |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 13            | 14            |
| Cash | Level 1                                                                                                |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 13            | 14            |
| Cash | Level 2                                                                                                |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 0             | 0             |
| Cash | Level 3                                                                                                |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 0             | 0             |
| Total assets measured at fair value                                                                           |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 1,357         | 1,317         |
| Total assets measured at fair value | Level 1                                                                 |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 441           | 388           |
| Total assets measured at fair value | Level 2                                                                 |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 906           | 919           |
| Total assets measured at fair value | Level 3                                                                 |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Fair value of plan assets                                                                                     | 10            | 10            |
| Total limited partnerships                                                                                    |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                                                              |          |          |
| Other assets                                                                                                  | $ 904         | $ 876         |

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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920
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