Reportable Segments - Schedule of Reportable Segments Revenue Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information
Insurance premiums									$ 52,516	$ 47,736	$ 44,052
Property-Liability net investment income									2,478	2,403	3,293
Total revenues	$ 14,832	$ 14,497	$ 13,979	$ 13,786	$ 13,648	$ 13,208	$ 12,219	$ 12,336	57,094	51,411	50,601
Operating segments | Property-Liability
Segment Reporting Information
Insurance premiums									48,427	43,909	40,454
Other revenue									1,545	1,416	1,437
Property-Liability net investment income									2,218	2,190	3,118
Net gains (losses) on investments and derivatives									(292)	(877)	1,021
Total revenues									51,898	46,638	46,030
Operating segments | Property-Liability | Allstate Protection
Segment Reporting Information
Insurance premiums									48,427	43,909	40,454
Operating segments | Property-Liability | Run-off Property-Liability
Segment Reporting Information
Insurance premiums									0	0	0
Operating segments | Property-Liability | Auto | Allstate Protection
Segment Reporting Information
Insurance premiums									32,940	29,715	27,623
Operating segments | Property-Liability | Homeowners | Allstate Protection
Segment Reporting Information
Insurance premiums									11,739	10,418	9,552
Operating segments | Property-Liability | Other personal lines | Allstate Protection
Segment Reporting Information
Insurance premiums									2,387	2,159	2,077
Operating segments | Property-Liability | Commercial lines | Allstate Protection
Segment Reporting Information
Insurance premiums									811	1,123	827
Operating segments | Property-Liability | Other business lines | Allstate Protection
Segment Reporting Information
Insurance premiums									550	494	375
Operating segments | Protection Services
Segment Reporting Information
Other revenue									319	347	354
Intersegment premiums and service fees									138	149	175
Property-Liability net investment income									73	48	43
Net gains (losses) on investments and derivatives									0	(52)	25
Total revenues									2,773	2,487	2,361
Operating segments | Protection Services | Protection Plans
Segment Reporting Information
Other revenue									1,540	1,307	1,132
Operating segments | Protection Services | Roadside assistance
Segment Reporting Information
Other revenue									195	202	192
Operating segments | Protection Services | Protection and insurance products
Segment Reporting Information
Other revenue									508	486	440
Operating segments | Allstate Health and Benefits
Segment Reporting Information
Other revenue									447	402	359
Property-Liability net investment income									82	69	74
Net gains (losses) on investments and derivatives									3	(44)	7
Total revenues									2,378	2,259	2,274
Operating segments | Allstate Health and Benefits | Employer voluntary benefits
Segment Reporting Information
Other revenue									1,001	1,033	1,040
Operating segments | Allstate Health and Benefits | Group health
Segment Reporting Information
Other revenue									440	385	350
Operating segments | Allstate Health and Benefits | Individual health
Segment Reporting Information
Other revenue									405	414	444
Operating segments | Corporate and Other
Segment Reporting Information
Other revenue									89	179	22
Property-Liability net investment income									105	96	58
Net gains (losses) on investments and derivatives									(11)	(99)	31
Total revenues									183	176	111
Intersegment eliminations
Segment Reporting Information
Total revenues									$ (138)	$ (149)	$ (175)