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Reinsurance and Indemnification - 2017-1 Excess Catastrophe Reinsurance Contract (Details) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) state
Ceded Credit Risk [Line Items] |  |
Retention amount for catastrophe reinsurance agreement contracts |  | $ 500
2017-1 Excess Catastrophe Reinsurance Contract |  |
Ceded Credit Risk [Line Items] |  |
Number of states covered by reinsurance agreement | state |  | 48
Limit of aggregate losses under aggregate excess agreement | $ 571 |
Retention percentage of reinsurance limit | 65.63% |
Minimum | 2017-1 Excess Catastrophe Reinsurance Contract |  |
Ceded Credit Risk [Line Items] |  |
Retention amount for catastrophe reinsurance agreement contracts | $ 3,400 |
Maximum | 2017-1 Excess Catastrophe Reinsurance Contract |  |
Ceded Credit Risk [Line Items] |  |
Retention amount for catastrophe reinsurance agreement contracts | $ 3,970 |

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- Definition Represents the retention percentage of annual coverage limit provided by the insurance arrangement for catastrophe claims. + References No definition available. + Details
Name: | all_CatastropheInsuranceAnnualCoverageLimitRetentionPercentage
Namespace Prefix: | all_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition The limit of aggregated losses per contract year beyond which coverage provided by an aggregate excess agreement applies. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Represents the retention amount of catastrophe reinsurance agreement contracts. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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- Definition Represents the number of states covered under the reinsurance agreement. + References No definition available. + Details
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- 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. + Details
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- Details
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- Details
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- Details
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