Variable Interest Entities - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Skyland
Variable Interest Entity [Line Items]
Quota share (as a percent)	100.00%
Adirondack
Variable Interest Entity [Line Items]
Quota share (as a percent)	36.50%
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Management fee	$ 10	$ 11
Notes payable	123		$ 123
Valuation allowance	123
Variable Interest Entity, Primary Beneficiary | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Ceded losses incurred	12	7
Variable Interest Entity, Primary Beneficiary | Allstate Protection
Variable Interest Entity [Line Items]
Insurance premiums and contract charges	61	57
Costs and expenses	$ 87	$ 59