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Reinsurance - Sanders and New Jersey Reinsurance Agreement (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) contract reinstatement
Ceded Credit Risk [Line Items] |
Retention amount for catastrophe reinsurance agreement contracts | $ 500
Sanders Re 2014-2 contract |
Ceded Credit Risk [Line Items] |
Limit of aggregate losses under aggregate excess agreement | 200
Retention amount for catastrophe reinsurance agreement contracts | $ 20
Term of catastrophe reinsurance agreement contract | 3 years
New Jersey Agreement |
Ceded Credit Risk [Line Items] |
Limit of aggregate losses under aggregate excess agreement | $ 400
Contracts number | contract | 3
New Jersey Agreement Contract One |
Ceded Credit Risk [Line Items] |
Retention amount for catastrophe reinsurance agreement contracts | $ 150
Retention percentage of reinsurance limit | 31.67%
Number of reinstatements each year for each contract | reinstatement | 1
New Jersey Agreement Contract Two |
Ceded Credit Risk [Line Items] |
Retention amount for catastrophe reinsurance agreement contracts | $ 144
Retention percentage of reinsurance limit | 31.67%
Number of reinstatements each year for each contract | reinstatement | 1
New Jersey Agreement Contract Three |
Ceded Credit Risk [Line Items] |
Retention amount for catastrophe reinsurance agreement contracts | $ 165
Retention percentage of reinsurance limit | 31.66%
Number of reinstatements each year for each contract | reinstatement | 1

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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
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the number of reinstatements each year for each contract. + References No definition available. + Details
Name: | all_CatastropheInsuranceNumberOfReinstatementsPerYear
Namespace Prefix: | all_
Data Type: | xbrli:integerItemType
Balance Type: | na
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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
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- Definition Represents the period of catastrophe reinsurance agreement contract. + References No definition available. + Details
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Data Type: | xbrli:durationItemType
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
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- Definition Represents the number of reinsurance contracts under the agreement. + References No definition available. + Details
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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
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- Details
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- Details
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- Details
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- Details
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- Details
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