Investments - Carrying value of non-impaired mortgage loans summarized by debt service coverage ratio distribution (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Total	$ 4,592	$ 4,534
Mortgage loans, non-impaired
Financing Receivable, Impaired [Line Items]
Fixed rate mortgage loans	4,457	4,438
Variable rate mortgage loans	131	92
Total	4,588	4,530
Below 1.0 \| Mortgage loans, non-impaired
Financing Receivable, Impaired [Line Items]
Fixed rate mortgage loans	2	3
Variable rate mortgage loans	30	0
Total	32	3
1.0 - 1.25 \| Mortgage loans, non-impaired
Financing Receivable, Impaired [Line Items]
Fixed rate mortgage loans	218	345
Variable rate mortgage loans	0	0
Total	218	345
1.26 - 1.50 \| Mortgage loans, non-impaired
Financing Receivable, Impaired [Line Items]
Fixed rate mortgage loans	1,216	1,141
Variable rate mortgage loans	0	30
Total	1,216	1,171
Above 1.50 \| Mortgage loans, non-impaired
Financing Receivable, Impaired [Line Items]
Fixed rate mortgage loans	3,021	2,949
Variable rate mortgage loans	101	62
Total	$ 3,122	$ 3,011

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.

No definition available.

+ Details

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

- Definition

Amount of loan with fixed rate of interest.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

+ Details

Name:	us-gaap_LoansReceivableWithFixedRatesOfInterest1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of loan with variable or adjustable rate of interest.

+ References

+ Details

Name:	us-gaap_LoansReceivableWithVariableRatesOfInterest1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.

+ 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 210.7-03.1(c))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=all_MortgagesNonImpairedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=all_DebtServiceCoverageRatioBelow1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=all_DebtServiceCoverageRatioBelow1To1.25Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=all_DebtServiceCoverageRatioBelow1.26To1.5Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=all_DebtServiceCoverageRatioBelowAbove1.5Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: