Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Jun. 30, 2020
Segment Reporting Information
Net investment income	$ 421	$ 648
Net income applicable to common shareholders	513	1,261
Shelter-in-Place Payback expense	210	0
Allstate Protection (1)
Segment Reporting Information
Underwriting income (loss)	1,348	703
Discontinued Lines and Coverages
Segment Reporting Information
Underwriting income (loss)	(3)	(3)
Property-Liability
Segment Reporting Information
Underwriting income (loss)	1,345	700
Net investment income	202	291
Income tax expense on operations	(303)	(202)
Realized capital gains (losses), after-tax	(82)	393
Net income applicable to common shareholders	1,162	1,182
Service Businesses
Segment Reporting Information
Adjusted net income	37	11
Net investment income	10	9
Realized capital gains (losses), after-tax	(19)	7
Amortization of purchased intangibles, after-tax	(21)	(24)
Net income applicable to common shareholders	(3)	(6)
Allstate Life
Segment Reporting Information
Adjusted net income	80	73
Net investment income	128	127
Realized capital gains (losses), after-tax	(25)	(4)
Valuation changes on embedded derivatives not hedged, after-tax	12	0
DAC and DSI amortization related to realized capital gains and losses, after-tax	(3)	(2)
Net income applicable to common shareholders	64	67
Allstate Benefits
Segment Reporting Information
Adjusted net income	24	31
Net investment income	20	19
Realized capital gains (losses), after-tax	(10)	3
Net income applicable to common shareholders	14	34
Allstate Annuities
Segment Reporting Information
Adjusted net income	(139)	(25)
Net investment income	47	190
Realized capital gains (losses), after-tax	(213)	124
Valuation changes on embedded derivatives not hedged, after-tax	2	(3)
Gain on disposition of operations, after-tax	1	1
Net income applicable to common shareholders	(349)	97
Corporate and Other
Segment Reporting Information
Adjusted net income	(107)	(103)
Realized capital gains (losses), after-tax	(17)	1
Pension and other postretirement remeasurement gains (losses), after-tax	(251)	(11)
Net income applicable to common shareholders	$ (375)	$ (113)
Forecast
Segment Reporting Information
Shelter-in-Place Payback expense			$ 600

Amortization Of Intangible Assets, Net Of Tax

No definition available.

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

- Definition

The amount of deferred policy acquisition costs and sales inducement costs (amortization) accretion related to realized capital gains and losses and valuation changes on embedded derivatives that are not hedged, net of tax effect.

+ References

No definition available.

+ Details

Name:	all_DeferredPolicyAcquisitionAndSalesInducementsCostsAmortizationAccretionRelatedToRealizedCapitalGainsLossesAndValuationChangesOnEmbeddedDerivativesNotHedgedAfterTax
Namespace Prefix:	all_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Defined Benefit Plan, Remeasurement Gain (Loss), After Tax

+ References

No definition available.

+ Details

Name:	all_DefinedBenefitPlanRemeasurementGainLossAfterTax
Namespace Prefix:	all_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The gain (loss) resulting from the sale of a disposal group that is not a discontinued operation, net of tax effect.

+ References

No definition available.

+ Details

Name:	all_DisposalGroupNotDiscontinuedOperationGainLossOnDisposalAfterTax
Namespace Prefix:	all_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The income tax expense (benefit) pertaining to operating income or loss as measured by the entity for segment reporting purposes.

+ References

No definition available.

+ Details

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

- Definition

The operating income (loss), net of tax effect, as measured by the entity for segment reporting purposes.

+ References

No definition available.

+ Details

Name:	all_OperatingIncomeLossAfterTax
Namespace Prefix:	all_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Policyholder Benefits, Shelter-in-Place Payback

+ References

No definition available.

+ Details

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

- Definition

The net realized gains or losses on investments during the period after tax effect.

+ References

No definition available.

+ Details

Name:	all_RealizedInvestmentGainsLossesAfterTax
Namespace Prefix:	all_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A measure that is not based on GAAP, which is calculated as premiums earned, less claims and claims expense ("losses"), amortization of deferred policy acquisition costs ("DAC"), operating costs and expenses and restructuring and related charges, as determined using GAAP.

+ References

No definition available.

+ Details

Name:	all_UnderwritingGainLoss
Namespace Prefix:	all_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of valuation changes on embedded derivatives that are not hedged, net of tax effect.

+ References

No definition available.

+ Details

Name:	all_ValuationChangesOnEmbeddedDerivativesNotHedgedAfterTax
Namespace Prefix:	all_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=117326831&loc=d3e1377-109256

+ Details

Name:	us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
-URI http://asc.fasb.org/extlink&oid=116637232&loc=SL114874131-224263

+ Details

Name:	us-gaap_NetInvestmentIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_SegmentReportingInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=all_AllStateProtectionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=all_DiscontinuedLinesAndCoveragesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=all_ServiceBusinessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=all_AllstateLifeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=all_AllstateBenefitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=all_AllstateAnnuitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateAndOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: