Reinsurance and Indemnification - New Jersey Excess Catastrophe Reinsurance Agreement (Details) - New Jersey Agreement $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($) contract reinstatement
Reinsurance Retention Policy [Line Items]
Contracts number \| contract	2
Retention percentage of reinsurance limit	63.00%
Limit of aggregate losses under aggregate excess agreement	$ 400
Retention amount for catastrophe reinsurance agreement contracts	$ 150
Number of reinstatements each year for each contract \| reinstatement	1

- Definition

Represents the retention percentage of annual coverage limit provided by the insurance arrangement for catastrophe claims.

+ References

No definition available.

+ Details

Name:	all_CatastropheInsuranceAnnualCoverageLimitRetentionPercentage
Namespace Prefix:	all_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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
Period Type:	duration

- Definition

The limit of aggregated losses per contract year beyond which coverage provided by an aggregate excess agreement applies.

+ References

No definition available.

+ Details

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

- Definition

Represents the retention amount of catastrophe reinsurance agreement contracts.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of reinsurance contracts under the agreement.

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No definition available.

+ Details

Name:	all_ReinsuranceContractsNumber
Namespace Prefix:	all_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- 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

Name:	us-gaap_ReinsuranceRetentionPolicyLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_CededCreditRiskAxis=all_NewJerseyAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: