Reportable Segments - Schedule of Reportable Segments Revenue Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information
Net investment income	$ 854	$ 764
Net gains (losses) on investments and derivatives	(349)	(164)
Total revenues	16,452	15,259
Operating segments | Property-Liability
Segment Reporting Information
Insurance premiums	14,027	12,900
Other revenue	488	430
Net investment income	783	702
Net gains (losses) on investments and derivatives	(329)	(162)
Total revenues	14,969	13,870
Operating segments | Property-Liability | Allstate Protection
Segment Reporting Information
Insurance premiums	14,027	12,900
Operating segments | Property-Liability | Run-off Property-Liability
Segment Reporting Information
Insurance premiums	0	0
Operating segments | Property-Liability | Auto | Allstate Protection
Segment Reporting Information
Insurance premiums	9,347	8,778
Operating segments | Property-Liability | Homeowners | Allstate Protection
Segment Reporting Information
Insurance premiums	3,657	3,154
Operating segments | Property-Liability | Other personal lines | Allstate Protection
Segment Reporting Information
Insurance premiums	741	659
Operating segments | Property-Liability | Commercial lines | Allstate Protection
Segment Reporting Information
Insurance premiums	113	169
Operating segments | Property-Liability | Other business lines | Allstate Protection
Segment Reporting Information
Insurance premiums	169	140
Operating segments | Protection Services
Segment Reporting Information
Other revenue	128	85
Intersegment premiums and service fees	37	35
Net investment income	24	21
Net gains (losses) on investments and derivatives	(10)	(5)
Total revenues	850	748
Operating segments | Protection Services | Protection plans
Segment Reporting Information
Insurance premiums	510	439
Operating segments | Protection Services | Roadside assistance
Segment Reporting Information
Insurance premiums	36	47
Operating segments | Protection Services | Protection and insurance products
Segment Reporting Information
Insurance premiums	125	126
Operating segments | Allstate Health and Benefits
Segment Reporting Information
Other revenue	131	134
Net investment income	25	23
Net gains (losses) on investments and derivatives	(1)	2
Total revenues	642	637
Operating segments | Allstate Health and Benefits | Employer voluntary benefits
Segment Reporting Information
Insurance premiums	243	248
Operating segments | Allstate Health and Benefits | Group health
Segment Reporting Information
Insurance premiums	124	118
Operating segments | Allstate Health and Benefits | Individual health
Segment Reporting Information
Insurance premiums	120	112
Operating segments | Corporate and Other
Segment Reporting Information
Other revenue	15	20
Net investment income	22	18
Net gains (losses) on investments and derivatives	(9)	1
Total revenues	28	39
Intersegment eliminations
Segment Reporting Information
Total revenues	$ (37)	$ (35)