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Subsequent Events (Details) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | ---
Oct. 31, 2017 USD ($) segment | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) segment | Sep. 30, 2016 USD ($) | Jan. 03, 2017 USD ($) | Dec. 31, 2016 USD ($)
Subsequent Event [Line Items] |  |  |  |  |  |  |  |
Number of reportable segments | segment |  |  |  |  | 4 |  |  |
Goodwill |  |  | $ 2,309 |  | $ 2,309 |  |  | $ 1,219
Change in unrealized net capital gains and losses |  |  | 125 | $ 193 | 598 | $ 1,197 |  |
Subsequent Event |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |
Number of reportable segments | segment | 7 |  |  |  |  |  |  |
Allstate Life | Subsequent Event |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |
Goodwill | $ 170 |  |  |  |  |  |  |
Allstate Benefits | Subsequent Event |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |
Goodwill | 100 |  |  |  |  |  |  |
Allstate Annuities | Subsequent Event |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |
Goodwill | $ 125 |  |  |  |  |  |  |
Scenario, Forecast |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |
Increase in future policy benefit reserves |  | $ 550 |  |  |  |  |  |
Change in unrealized net capital gains and losses |  | (350) |  |  |  |  |  |
Scenario, Forecast | Allstate Annuities |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |
Goodwill, impairment loss |  | $ 125 |  |  |  |  |  |
SquareTrade |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |
Goodwill |  |  | $ 1,090 |  | $ 1,090 |  | $ 1,080 |

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