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Benefit Plans - Allstate 401(k) savings plan (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Retirement Benefits [Abstract] |  |  |
Company's contribution to the Allstate Plan | $ 103 | $ 93 | $ 89
ESOP benefit |  | (41) | (2)
Principal balance of note from ESOP |  | 2 |
Defined contribution plan expense for eligible employees of Canadian insurance subsidiaries and Sterling | $ 13 | $ 15 | $ 15

X
- Definition This element represents the defined contribution plans expenses for eligible employees of certain subsidiaries. + References No definition available. + Details
Name: | all_DefinedContributionPlanExpenseForEligibleEmployeesOfCertainSubsidiaries
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition The element represents the aggregate amount of ESOP benefit. + References No definition available. + Details
Name: | all_EmployeeStockOwnershipPlanESOPBenefit
Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

X
- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
Name: | us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The amount reported as a liability as of the balance sheet date by the entity (the employer) of the loan made by a third-party lender directly to the ESOP. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 25 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109244457&loc=d3e16545-113920 + Details
Name: | us-gaap_EmployeeStockOwnershipPlanESOPDebtStructureDirectLoanAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant