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Reinsurance - Catastrophe reinsurance (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) contract
Ceded Credit Risk [Line Items] |
Retention amount for catastrophe reinsurance agreement contracts | $ 500
Nationwide Per Occurrence Excess Catastrophe Reinsurance Program |
Ceded Credit Risk [Line Items] |
Limit of aggregate losses under aggregate excess agreement | 4,420
Retention amount for catastrophe reinsurance agreement contracts | $ 500
Catastrophe reinsurance, number of agreements | contract | 3

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

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- Definition The number of separate catastrophe reinsurance agreements coordinating coverage for personal property excess catastrophe losses. + References No definition available. + Details
Name: | all_CatastropheReinsuranceNumberOfAgreements
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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |