Reinsurance and Indemnification - Canada Catastrophe Excess of Loss Reinsurance Contract (Narrative) (Details) $ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2025 USD ($) reinstatement	Dec. 31, 2025 CAD ($) reinstatement	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Reinsurance Retention Policy [Line Items]
Property-liability insurance premiums earned	$ 2,681		$ 2,260	$ 2,036
Catastrophe Reinsurance Program
Reinsurance Retention Policy [Line Items]
Property-liability insurance premiums earned	$ 1,230		$ 1,110	$ 1,020
Canada Catastrophe Excess of Loss \| Catastrophe Reinsurance Program
Reinsurance Retention Policy [Line Items]
Limit of aggregate losses under aggregate excess agreement		$ 478
Retention amount for catastrophe reinsurance agreement contracts		$ 100
Number of reinstatements each year for each contract \| reinstatement	1	1

Amount of earned premiums ceded to other entities.

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+ Details

ALLSTATE CORP 10-K