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Summary of Significant Accounting Policies - Goodwill (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Goodwill |  |  |  |
Goodwill allocated to segments |  | $ 2,545 | $ 2,530 |
Impairment of goodwill and purchased intangibles | $ 125 | 106 | 0 | $ 125
Allstate Protection |  |  |  |
Goodwill |  |  |  |
Goodwill allocated to segments |  | 810 | 810 |
Service Businesses |  |  |  |
Goodwill |  |  |  |
Goodwill allocated to segments |  | 1,464 | 1,449 |
Allstate Life |  |  |  |
Goodwill |  |  |  |
Goodwill allocated to segments |  | 175 | 175 |
Allstate Benefits |  |  |  |
Goodwill |  |  |  |
Goodwill allocated to segments |  | $ 96 | $ 96 |

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