| **Investments (Summary of available-for-sale commercial mortgage-backed securities in gross unrealized loss position by property type and rating) (Details) - USD ($) $ in Millions** | Mar. 31, 2020 | Dec. 31, 2019 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                                  |          |          |
| Estimated Fair Value                                                                                                                                                                  | $ 10,153      | $ 2,420       |
| Gross Unrealized Losses                                                                                                                                                               | 1,117         | 45            |
| Commercial mortgage-backed                                                                                                                                                            |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                                  |          |          |
| Estimated Fair Value                                                                                                                                                                  | 1,197         | 401           |
| Gross Unrealized Losses                                                                                                                                                               | 153           | $ 5           |
| Conduits (multi property, multi borrower pools) | Commercial mortgage-backed                                                                                                          |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                                  |          |          |
| Estimated Fair Value                                                                                                                                                                  | 211           |          |
| Gross Unrealized Losses                                                                                                                                                               | 11            |          |
| Single asset, single borrower | Commercial mortgage-backed                                                                                                                            |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                                  |          |          |
| Estimated Fair Value                                                                                                                                                                  | 986           |          |
| Gross Unrealized Losses                                                                                                                                                               | 142           |          |
| US Government, Government agencies and Government sponsored enterprises | Commercial mortgage-backed                                                                                  |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                                  |          |          |
| Estimated Fair Value                                                                                                                                                                  | 1             |          |
| Gross Unrealized Losses                                                                                                                                                               | 0             |          |
| AAA | Commercial mortgage-backed                                                                                                                                                      |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                                  |          |          |
| Estimated Fair Value                                                                                                                                                                  | 60            |          |
| Gross Unrealized Losses                                                                                                                                                               | 1             |          |
| AA | Commercial mortgage-backed                                                                                                                                                       |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                                  |          |          |
| Estimated Fair Value                                                                                                                                                                  | 245           |          |
| Gross Unrealized Losses                                                                                                                                                               | 17            |          |
| A | Commercial mortgage-backed                                                                                                                                                        |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                                  |          |          |
| Estimated Fair Value                                                                                                                                                                  | 214           |          |
| Gross Unrealized Losses                                                                                                                                                               | 22            |          |
| BBB | Commercial mortgage-backed                                                                                                                                                      |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                                  |          |          |
| Estimated Fair Value                                                                                                                                                                  | 484           |          |
| Gross Unrealized Losses                                                                                                                                                               | 78            |          |
| Below investment grade | Commercial mortgage-backed                                                                                                                                   |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                                  |          |          |
| Estimated Fair Value                                                                                                                                                                  | 193           |          |
| Gross Unrealized Losses                                                                                                                                                               | $ 35          |          |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
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| - Definition
Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.
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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.
+ References
No definition available.
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