Reinsurance and indemnification - Rollforward of credit loss allowance for reinsurance recoverables (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Property-Liability
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	$ (59)	$ (60)
Increase (decrease) in the provision for credit losses	(1)	1
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	(60)	(59)
Allstate Health and Benefits
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	(1)	(1)
Increase (decrease) in the provision for credit losses	0	0
Write-offs	0	0
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	$ (1)	$ (1)

Amount of the write-down or write-off of a receivable from a reinsurer.

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- Definition

Amount of increase (decrease) in allowance for credit loss on reinsurance recoverable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

+ Details

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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- Definition

Amount of allowance for credit loss on reinsurance recoverable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446

+ Details

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- Details

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- Details

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