Reportable Segments - Schedule of Reportable Segments Total Assets, Investments and Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Segment Reporting Information
Assets	$ 120,402	$ 111,617
Investments	82,328	72,610
Deferred policy acquisition costs	6,095	5,773
Other investments, net	1,017	824
All other
Segment Reporting Information
Assets	974	648
Investments	197	160
Deferred policy acquisition costs	81	63
Allstate Protection and Run-off Property-Liability | Operating segments
Segment Reporting Information
Assets	105,668	96,988
Investments	74,390	67,671
Allstate Protection and Run-off Property-Liability | Operating segments | Allstate Protection
Segment Reporting Information
Deferred policy acquisition costs	2,802	2,548
Protection Services | Operating segments
Segment Reporting Information
Assets	8,442	7,540
Investments	2,501	2,228
Deferred policy acquisition costs	3,212	3,161
Allstate Health and Benefits | Operating segments
Segment Reporting Information
Assets	0	3,714
Investments	0	219
Deferred policy acquisition costs	0	1
Allstate Health and Benefits | Disposal Group, Held-for-Sale, Not Discontinued Operations
Segment Reporting Information
Other investments, net		1,910
Deferred policy acquisition costs		521
Corporate | Operating segments
Segment Reporting Information
Assets	5,318	2,727
Investments	$ 5,240	$ 2,332