Reportable Segments - Schedule of Reportable Segments Revenue Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information
Net investment income	$ 949	$ 783	$ 2,557	$ 2,259
Net gains (losses) on investments and derivatives	252	243	(241)	(24)
Total revenues	17,255	16,627	50,340	47,600
Operating segments | Property Liability
Segment Reporting Information
Insurance premiums	15,051	14,225	44,416	41,335
Other revenue	518	531	1,510	1,402
Net investment income	873	708	2,343	2,053
Net gains (losses) on investments and derivatives	157	222	(362)	(43)
Operating segments | Property Liability | Total Allstate Protection insurance premiums
Segment Reporting Information
Insurance premiums	14,533	13,694	42,906	39,933
Operating segments | Property Liability | Run-off Property-Liability
Segment Reporting Information
Insurance premiums	0	0	0	0
Operating segments | Property Liability | Auto | Total Allstate Protection insurance premiums
Segment Reporting Information
Insurance premiums	9,593	9,270	28,468	27,127
Operating segments | Property Liability | Homeowners | Total Allstate Protection insurance premiums
Segment Reporting Information
Insurance premiums	3,880	3,403	11,308	9,812
Operating segments | Property Liability | Other personal lines | Total Allstate Protection insurance premiums
Segment Reporting Information
Insurance premiums	800	718	2,320	2,078
Operating segments | Property Liability | Commercial lines | Total Allstate Protection insurance premiums
Segment Reporting Information
Insurance premiums	99	151	316	478
Operating segments | Property Liability | Other business lines | Total Allstate Protection insurance premiums
Segment Reporting Information
Insurance premiums	161	152	494	438
Operating segments | Protection Services
Segment Reporting Information
Intersegment premiums and service fees	33	49	106	123
Other revenue	124	110	363	293
Net investment income	25	24	74	68
Net gains (losses) on investments and derivatives	10	10	1	4
Total revenues	912	832	2,630	2,352
Operating segments | Protection Services | Protection plans
Segment Reporting Information
Insurance premiums	552	480	1,593	1,372
Operating segments | Protection Services | Roadside assistance
Segment Reporting Information
Insurance premiums	42	34	117	115
Operating segments | Protection Services | Protection and insurance products
Segment Reporting Information
Insurance premiums	126	125	376	377
Operating segments | Allstate Health and Benefits
Segment Reporting Information
Other revenue	0	81	163	245
Net investment income	0	25	24	69
Net gains (losses) on investments and derivatives	0	(6)	(1)	(3)
Total revenues	0	468	676	1,411
Operating segments | Allstate Health and Benefits | Employer voluntary benefits
Segment Reporting Information
Insurance premiums	0	248	243	742
Operating segments | Allstate Health and Benefits | Group health
Segment Reporting Information
Insurance premiums	0	120	247	358
Operating segments | Corporate
Segment Reporting Information
Other revenue	24	17	62	56
Net investment income	49	25	108	64
Net gains (losses) on investments and derivatives	85	17	122	19
Total revenues	158	59	292	139
All other
Segment Reporting Information
Net gains (losses) on investments and derivatives	252	243	(241)	(24)
Total revenues	137	162	451	476
All other | Property Liability
Segment Reporting Information
Net investment income	873	708	2,343	2,053
Intersegment eliminations
Segment Reporting Information
Total revenues	$ (33)	$ (49)	$ (106)	$ (123)