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Reinsurance - Claims expense, life contract benefits and interest credited to contractholder funds (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Property and casualty insurance claims and claims expense |  |
Reductions to costs and expenses due to reinsurance ceded amounts |  |
Ceded losses incurred | $ (187) | $ (131)
Life contract benefits |  |
Reductions to costs and expenses due to reinsurance ceded amounts |  |
Ceded losses incurred | (49) | (47)
Interest credited to contractholder funds |  |
Reductions to costs and expenses due to reinsurance ceded amounts |  |
Ceded losses incurred | $ (4) | $ (5)

X
- 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

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

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

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