| **Investments - Net realized investment gains (losses) (Details) - USD ($) $ in Millions** | 3 Months Ended |
|--------------------------------------------------------------------------------------------|----------------|
||
| **Schedule of Available-for-sale Securities [Line Items]**                                 |           |   |
| Net realized investment gains (losses) on fixed maturity securities                        | $ 36           | $ (36) |
| Fixed maturity securities                                                                  |           |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                 |           |   |
| Gross realized gains                                                                       | 49             | 45 |
| Gross realized losses                                                                      | (17)           | (62) |
| Net realized investment gains (losses) on fixed maturity securities                        | 32             | (17) |
| Equity securities                                                                          |           |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                 |           |   |
| Gross realized gains                                                                       | 0              | 0 |
| Gross realized losses                                                                      | 0              | (5) |
| Net realized investment gains (losses) on fixed maturity securities                        | 0              | (5) |
| Derivatives                                                                                |           |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                 |           |   |
| Net realized investment gains (losses) on fixed maturity securities                        | 1              | (7) |
| Short term investments and other                                                           |           |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                 |           |   |
| Net realized investment gains (losses) on fixed maturity securities                        | $ 3            | $ (7) |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563
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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                                                                                                                                                                           |
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| X                                                                                                                                                                      |
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| X                                                                                                                                                                |
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| X                                                                                                                                                                         |
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