Reportable Segments - Schedule of Reportable Segments Revenue Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information
Net investment income	$ 783	$ 689	$ 2,259	$ 1,874
Net gains (losses) on investments and derivatives	243	(86)	(24)	(223)
Consolidated revenues	16,627	14,497	47,600	42,262
Intersegment Eliminations
Segment Reporting Information
Consolidated revenues	(49)	(34)	(123)	(102)
Property-Liability | Operating Segments
Segment Reporting Information
Insurance premiums	13,694	12,270	39,933	35,826
Other revenue	531	393	1,402	1,135
Net investment income	708	627	2,053	1,680
Net gains (losses) on investments and derivatives	222	(62)	(43)	(185)
Consolidated revenues	15,155	13,228	43,345	38,456
Property-Liability | Operating Segments | Allstate Protection
Segment Reporting Information
Insurance premiums	13,694	12,270	39,933	35,826
Property-Liability | Operating Segments | Run-off Property-Liability
Segment Reporting Information
Insurance premiums	0	0	0	0
Property-Liability | Operating Segments | Auto | Allstate Protection
Segment Reporting Information
Insurance premiums	9,270	8,345	27,127	24,374
Property-Liability | Operating Segments | Homeowners | Allstate Protection
Segment Reporting Information
Insurance premiums	3,403	2,969	9,812	8,662
Property-Liability | Operating Segments | Other personal lines | Allstate Protection
Segment Reporting Information
Insurance premiums	718	608	2,078	1,757
Property-Liability | Operating Segments | Commercial lines | Allstate Protection
Segment Reporting Information
Insurance premiums	151	194	478	628
Property-Liability | Operating Segments | Other business lines | Allstate Protection
Segment Reporting Information
Insurance premiums	152	154	438	405
Protection Services | Operating Segments
Segment Reporting Information
Other revenue	110	75	293	243
Net investment income	24	19	68	53
Net gains (losses) on investments and derivatives	10	(8)	4	(13)
Intersegment premiums and service fees	49	34	123	102
Consolidated revenues	832	689	2,352	2,041
Protection Services | Operating Segments | Protection plans
Segment Reporting Information
Other revenue	480	392	1,372	1,126
Protection Services | Operating Segments | Roadside assistance
Segment Reporting Information
Other revenue	34	51	115	148
Protection Services | Operating Segments | Protection and insurance products
Segment Reporting Information
Other revenue	125	126	377	382
Allstate Health and Benefits | Operating Segments
Segment Reporting Information
Other revenue	123	104	378	306
Net investment income	26	20	74	60
Net gains (losses) on investments and derivatives	(6)	(2)	(4)	1
Consolidated revenues	630	585	1,887	1,746
Allstate Health and Benefits | Operating Segments | Employer voluntary benefits
Segment Reporting Information
Other revenue	248	253	742	753
Allstate Health and Benefits | Operating Segments | Group health
Segment Reporting Information
Other revenue	120	111	358	328
Allstate Health and Benefits | Operating Segments | Individual health
Segment Reporting Information
Other revenue	119	99	339	298
Corporate and Other | Operating Segments
Segment Reporting Information
Other revenue	17	20	56	66
Net investment income	25	23	64	81
Net gains (losses) on investments and derivatives	17	(14)	19	(26)
Consolidated revenues	$ 59	$ 29	$ 139	$ 121