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Reserve for Property and Casualty Insurance Claims and Claims Expense - Reserves for asbestos, environmental and other discontinued lines claims before and after the effects of reinsurance (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Asbestos claims |  |
Gross reserves | $ 1,204 | $ 1,172
Reinsurance | (377) | (362)
Net reserves | 827 | 810
Environmental claims |  |
Gross reserves | 249 | 219
Reinsurance | (43) | (40)
Net reserves | 206 | 179
Other discontinued lines |  |
Gross reserves | 435 | 427
Reinsurance | (60) | (51)
Net reserves | 375 | 376
Total |  |
Gross reserves | 1,888 | 1,818
Reinsurance | (480) | (453)
Net reserves | $ 1,408 | $ 1,365

X
- Definition Asbestos claims + References No definition available. + Details
Name: | all_AsbestosClaimsAbstract
Namespace Prefix: | all_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

X
- Definition Liability for Asbestos Claims Gross + References No definition available. + Details
Name: | all_LiabilityForAsbestosClaimsGross
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos, net of estimated reinsurance recoveries. + References No definition available. + Details
Name: | all_LiabilityForAsbestosClaimsNet
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Liability for Environmental Claims Gross + References No definition available. + Details
Name: | all_LiabilityForEnvironmentalClaimsGross
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to environmental claims, net of estimated reinsurance recoveries. + References No definition available. + Details
Name: | all_LiabilityForEnvironmentalClaimsNet
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Liability for Other Discontinued Claims Gross + References No definition available. + Details
Name: | all_LiabilityForOtherDiscontinuedClaimsGross
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Other discontinued lines + References No definition available. + Details
Name: | all_OtherDiscontinuedLinesAbstract
Namespace Prefix: | all_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The known and estimated amount recoverable related to asbestos claims as of the balance sheet date from reinsurers for claims paid or incurred by the ceding insurer and associated claims settlement expenses, including estimated amounts for claims incurred but not reported, and policy benefits, net of any related valuation allowance. + References No definition available. + Details
Name: | all_ReinsuranceRecoverablesAsbestos
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The known and estimated amount recoverable related to environment related claims as of the balance sheet date from reinsurers for claims paid or incurred by the ceding insurer and associated claims settlement expenses, including estimated amounts for claims incurred but not reported, and policy benefits, net of any related valuation allowance. + References No definition available. + Details
Name: | all_ReinsuranceRecoverablesEnvironment
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Reinsurance Recoverables Other Discontinued + References No definition available. + Details
Name: | all_ReinsuranceRecoverablesOtherDiscontinued
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Reserves for Other Discontinued Lines + References No definition available. + Details
Name: | all_ReservesForOtherDiscontinuedLines
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Total Liability Claims Gross + References No definition available. + Details
Name: | all_TotalLiabilityClaimsGross
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Total Liability Claims Net + References No definition available. + Details
Name: | all_TotalLiabilityClaimsNet
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Total Reinsurance Recoverables + References No definition available. + Details
Name: | all_TotalReinsuranceRecoverables
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_ReinsuranceRecoverablesAlternativeAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration