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Reinsurance and Indemnification - Other reinsurance programs (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Reinsurance Retention Policy [Line Items] |  |
Reinsurance and indemnification recoverables, net | $ 9,565 | $ 8,921
Lloyds of London |  |
Reinsurance Retention Policy [Line Items] |  |
Reinsurance and indemnification recoverables, net | 165 | 167
Property-Liability |  |
Reinsurance Retention Policy [Line Items] |  |
Reinsurance and indemnification recoverables, net | 7,270 | 6,570
Property-Liability | Westport Insurance Corporation |  |
Reinsurance Retention Policy [Line Items] |  |
Largest reinsurance recoverable from a single reinsurer | $ 60 | $ 61

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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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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://fasb.org/us-gaap/role/ref/legacyRef -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=6879938&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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- 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
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=all_PropertyLiabilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |