| **Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions** | 3 Months Ended | 12 Months Ended |
|-------------------------------------------------------------------------------------------------|----------------|-----------------|
||
| **Liability for Catastrophe Claims [Line Items]**                                               |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                        | $ 2,448        | $ 2,398         | $ 2,366 | $ 2,330 | $ 2,390 | $ 2,433 | $ 2,348 | $ 2,195 | $ 9,542 | $ 9,366 | $ 9,101 |
| Net income                                                                                      | $ 223          | $ 144           | $ 272 | $ 260 | $ 241 | $ 343 | $ 209 | $ 66 | $ 899 | $ 859 | $ 479 |
| Basic earnings (loss) per share (in usd per share)                                              | $ 0.82         | $ 0.53          | $ 1.01 | $ 0.96 | $ 0.89 | $ 1.27 | $ 0.77 | $ 0.25 | $ 3.32 | $ 3.18 | $ 1.77 |
| Diluted earnings (loss) per share (in usd per share)                                            | $ 0.82         | $ 0.53          | $ 1.00 | $ 0.96 | $ 0.89 | $ 1.26 | $ 0.77 | $ 0.24 | $ 3.30 | $ 3.17 | $ 1.77 |
| Catastrophe losses net of reinsurance                                                           |           |            |   |   |   |   |   |   | $ 380 | $ 165 | $ 141 |
| Net deferred tax asset remeasurement                                                            | $ (83)         |            |   |   |   |   |   |   | (83) | $ 0 | $ 0 |
| Hurricane Harvey, Irma and Maria                                                                |           |            |   |   |   |   |   |   |   |   |   |
| **Liability for Catastrophe Claims [Line Items]**                                               |           |            |   |   |   |   |   |   |   |   |   |
| Catastrophe losses net of reinsurance                                                           |           | $ 188           |   |   |   |   |   |   | $ 256 |   |   |

| X                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Catastrophe losses net of reinsurance
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Effective Income Tax Rate Reconciliation, Effect of Tax Cuts and Jobs Act of 2017, Amount
+ References
No definition available.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.
+ 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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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).
+ References
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| X                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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