Reportable Segments - Schedule of Reportable Segment Total Assets, Investments and Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024
Segment Reporting Information
Assets	$ 115,161	$ 111,617
Investments	74,050	72,610
Deferred policy acquisition costs	5,787	5,773	$ 5,946
Other investments, net	901	824
Property-Liability
Segment Reporting Information
Assets	100,875	96,988
Investments	69,573	67,671
Deferred policy acquisition costs	2,565	2,548
Protection Services
Segment Reporting Information
Assets	7,593	7,540
Investments	2,334	2,228
Deferred policy acquisition costs	3,152	3,161
Allstate Health and Benefits
Segment Reporting Information
Assets	4,319	4,362
Investments	187	379
Deferred policy acquisition costs	70	64
Allstate Health and Benefits | Disposal Group, Held-for-Sale, Not Discontinued Operations
Segment Reporting Information
Other investments, net	2,070	1,910
Deferred policy acquisitions costs	526	521
Corporate and Other
Segment Reporting Information
Assets	2,374	2,727
Investments	$ 1,956	$ 2,332