Variable Interest Entities - Narrative (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2025	Mar. 31, 2025	Sep. 30, 2024	Mar. 31, 2024	Sep. 30, 2025	Sep. 30, 2024
Variable Interest Entity [Line Items]
Net gains (losses) on investments and derivatives	$ 252		$ 243		$ (241)	$ (24)
Interests in surplus notes (in percent)	100.00%				100.00%
Skyland
Variable Interest Entity [Line Items]
Quota share (as a percent)	100.00%				100.00%
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Net gains (losses) on investments and derivatives		$ (67)		$ (123)
Management fee			2			23
Variable Interest Entity, Primary Beneficiary | Allstate Protection
Variable Interest Entity [Line Items]
Insurance premiums and contract charges	$ 0		48		$ (1)	170
Costs and expenses	3		62		10	207
Variable Interest Entity, Primary Beneficiary | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Ceded losses incurred	$ (1)		$ (6)		$ (1)	$ 24