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Reinsurance - Claims expense, life contract benefits and interest credited to contractholder funds (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Property and casualty insurance claims and claims expense |  |  |  |
Reductions to costs and expenses due to reinsurance ceded amounts |  |  |  |
Ceded losses incurred | $ (138) | $ (124) | $ (325) | $ (255)
Life contract benefits |  |  |  |
Reductions to costs and expenses due to reinsurance ceded amounts |  |  |  |
Ceded losses incurred | (66) | (70) | (115) | (117)
Interest credited to contractholder funds |  |  |  |
Reductions to costs and expenses due to reinsurance ceded amounts |  |  |  |
Ceded losses incurred | $ (7) | $ (6) | $ (11) | $ (11)

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- Definition Represents the amount of reinsurance ceded under various contracts. + References No definition available. + Details
Name: | all_ReinsuranceCededAmount
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
Name: | all_ReinsuranceClaimsBenefitsCededAbstract
Namespace Prefix: | all_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_IncomeStatementLocationAxis=all_PropertyLiabilityInsuranceClaimsAndClaimsExpenseMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_IncomeStatementLocationAxis=all_LifeAndAnnuityContractBenefitsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_IncomeStatementLocationAxis=all_InterestCreditedToContractholderFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |