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Reinsurance and Indemnification - Kentucky Earthquake Excess Catastrophe Reinsurance Contract (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2019 USD ($) contract limit
Reinsurance Retention Policy [Line Items] |
Limit of aggregate losses under aggregate excess agreement | $ 3,400
Kentucky Agreement |
Reinsurance Retention Policy [Line Items] |
Term of catastrophe reinsurance agreement contract | 3 years
Number of limits | limit | 3
Limit of aggregate losses under aggregate excess agreement | $ 28
Retention amount for catastrophe reinsurance agreement contracts | $ 2
Number of limits available per contract year | limit | 2
Contracts number | contract | 1
Retention percentage of reinsurance limit | 95.00%

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

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- Definition Represents the period of catastrophe reinsurance agreement contract. + References No definition available. + Details
Name: | all_CatastropheReinsuranceAgreementContractTerm
Namespace Prefix: | all_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the number of limits as per catastrophe reinsurance agreement contracts. + References No definition available. + Details
Name: | all_CatastropheReinsuranceAgreementContractsNumberofLimits
Namespace Prefix: | all_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Catastrophe Reinsurance Agreement Contracts, Number of Limits Available Per Contract Year + References No definition available. + Details
Name: | all_CatastropheReinsuranceAgreementContractsNumberofLimitsAvailablePerContractYear
Namespace Prefix: | all_
Data Type: | xbrli:integerItemType
Balance Type: | na
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 Represents the number of reinsurance contracts under the agreement. + References No definition available. + Details
Name: | all_ReinsuranceContractsNumber
Namespace Prefix: | all_
Data Type: | xbrli:integerItemType
Balance Type: | na
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_KentuckyAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |