Reportable Segments - Schedule of Reportable Segment Total Assets, Investments and Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information
Assets	$ 103,362	$ 97,989
Investments	66,677	61,829
Deferred policy acquisition costs	5,940	5,442	$ 4,738	$ 3,776
Property-Liability
Segment Reporting Information
Assets	88,568	82,744
Investments	59,540	54,011
Deferred policy acquisition costs	2,378	2,146
Protection Services
Segment Reporting Information
Assets	7,292	6,922
Investments	2,180	1,917
Deferred policy acquisition costs	3,022	2,768
Allstate Health and Benefits
Segment Reporting Information
Assets	4,032	3,752
Investments	2,182	1,872
Deferred policy acquisition costs	540	528
Corporate and Other
Segment Reporting Information
Assets	3,470	4,571
Investments	$ 2,775	$ 4,029