| **Investments (Net investment income) (Details) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|---------------------------------------------------------------------------|----------------|----------------|
||
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | $ 533          | $ 503          | $ 1,428 | $ 1,621 |
| Investment expense                                                        | (16)           | (16)           | (48) | (48) |
| Net investment income                                                     | 517            | 487            | 1,380 | 1,573 |
| Fixed maturity securities                                                 |           |           |   |   |
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | 432            | 452            | 1,300 | 1,362 |
| Equity securities                                                         |           |           |   |   |
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | 18             | 16             | 24 | 62 |
| Limited partnership investments                                           |           |           |   |   |
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | 64             | 12             | 38 | 125 |
| Mortgage loans                                                            |           |           |   |   |
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | 14             | 13             | 42 | 37 |
| Short term investments                                                    |           |           |   |   |
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | 1              | 8              | 9 | 27 |
| Trading portfolio                                                         |           |           |   |   |
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | 4              | 2              | 13 | 6 |
| Other                                                                     |           |           |   |   |
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | $ 0            | $ 0            | $ 2 | $ 2 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, before investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of expenses related to the generation of investment income.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.9) -URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.2) -URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(2)) -URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263
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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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