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Reinsurance and Indemnification - Excess contract (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($)
Reinsurance Retention Policy [Line Items] |
Retention amount for catastrophe reinsurance agreement contracts | $ 500
Florida Excess Catastrophe Reinsurance Agreement, Personal Property Lines |
Reinsurance Retention Policy [Line Items] |
Limit of aggregate losses under aggregate excess agreement | 249
Retention amount for catastrophe reinsurance agreement contracts | $ 20
Retention percentage of reinsurance limit | 100.00%
Catastrophe reinsurance aggregate loss limit, exhausted | $ 42

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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

X
- Definition Catastrophe Reinsurance Aggregate Loss Limit, Exhausted + References No definition available. + Details
Name: | all_CatastropheReinsuranceAggregateLossLimitExhausted
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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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
Name: | all_CatastropheReinsuranceAggregateLossLimitPerContractYear
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represents the retention amount of catastrophe reinsurance agreement contracts. + References No definition available. + Details
Name: | all_CatastropheReinsuranceAgreementContractsRetentionAmount
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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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
Name: | us-gaap_ReinsuranceRetentionPolicyLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_CededCreditRiskAxis=all_FloridaExcessCatastropheReinsuranceAgreementPersonalPropertyLinesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |