Reinsurance and indemnification - Rollforward of credit loss allowance for reinsurance recoverables (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Property-Liability
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ (60)	$ (59)	$ (59)	$ (60)
Increase (decrease) in the provision for credit losses	(6)	0	(7)	1
Ending balance	(66)	(59)	(66)	(59)
Allstate Health and Benefits
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning balance	(1)	(1)	(1)	(1)
Increase (decrease) in the provision for credit losses	0	0	0	0
Write-offs	0	0	0	0
Ending balance	$ (1)	$ (1)	$ (1)	$ (1)