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Reinsurance and Indemnification - Other reinsurance programs (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Lloyds of London |  |
Reinsurance Retention Policy [Line Items] |  |
Reinsurance and indemnification recoverables, net | $ 166 | $ 158
Aleka Insurance Inc |  |
Reinsurance Retention Policy [Line Items] |  |
Largest reinsurance recoverable from a single reinsurer | $ 165 | $ 115

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- Definition Represents the amount of reported and estimated unreported claims assumed by and receivable from the identified reinsurer with respect to disclosure of a concentration of credit risk. + References No definition available. + Details
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Namespace Prefix: | all_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount, after valuation allowance, recoverable under reinsurance contracts including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(6)) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21409-158489 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | debit
Period Type: | instant

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- 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
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- Details
Name: | us-gaap_CededCreditRiskAxis=all_LloydsOfLondonMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_CededCreditRiskAxis=all_AlekaInsuranceIncMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |