Reportable Segments - Schedule of Reportable Segments Expense Information Used in Measure for Segment Profit or Loss (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
Prior year reserve reestimates	$ (424)	$ 32	$ (1,045)	$ (322)
Accident, health and other policy benefits	67	317	588	904
Amortization of DAC	2,101	2,037	6,264	5,977
Amortization of purchased intangibles	59	71	175	210
Interest expense	101	104	301	299
Restructuring and related charges	17	28	48	51
Income taxes on operations	1,075	254	1,802	603
Preferred stock dividends	29	29	88	88
Non-catastrophe losses
Segment Reporting Information
Prior year reserve reestimates	(396)	46	(1,007)	(8)
Catastrophe
Segment Reporting Information
Prior year reserve reestimates	(28)	(14)	(38)	(314)
Loss adjustment expenses, net of reinsurance			4,750	4,550
Operating segments | Allstate Protection | Allstate Protection
Segment Reporting Information
Property and casualty insurance claims and claims expense	8,304	8,501	25,467	25,701
Amortization of DAC	1,757	1,696	5,231	4,977
Advertising expense	575	519	1,540	1,204
Amortization of purchased intangibles	46	52	138	154
Restructuring and related charges	15	23	44	45
Other segment expenses	1,298	1,190	3,717	3,457
Total	12,011	13,670	39,729	40,019
Loss adjustment expenses, net of reinsurance	741	732	2,220	2,140
Operating segments | Allstate Protection | Allstate Protection | Non-catastrophe losses
Segment Reporting Information
Prior year reserve reestimates	(542)	(14)	(1,158)	(73)
Operating segments | Allstate Protection | Allstate Protection | Catastrophe
Segment Reporting Information
Property and casualty insurance claims and claims expense	558	1,703	4,750	4,554
Operating segments | Allstate Protection | Run-off Property-Liability
Segment Reporting Information
Property and casualty insurance claims and claims expense	146	59	151	65
Other segment expenses	0	1	2	3
Total	146	60	153	68
Loss adjustment expenses, net of reinsurance	24	4	28	7
Operating segments | Protection Services
Segment Reporting Information
Property and casualty insurance claims and claims expense	193	166	524	481
Amortization of DAC	337	304	983	889
Non-deferrable commissions	116	94	327	251
Restructuring and related charges	1	0	2	1
Other segment expenses	194	186	582	509
Income taxes on operations	16	15	51	51
Total	857	765	2,469	2,182
Operating segments | Allstate Health and Benefits
Segment Reporting Information
Accident, health and other policy benefits	0	255	379	723
Amortization of DAC	0	30	30	90
Restructuring and related charges	0	1	0	2
Other segment expenses	0	147	213	430
Income taxes on operations	0	9	12	36
Total	0	442	634	1,281
Operating segments | Corporate
Segment Reporting Information
Interest expense	101	104	301	299
Restructuring and related charges	0	3	0	2
Other segment expenses	42	39	119	128
Income taxes on operations	(12)	(23)	(60)	(77)
Preferred stock dividends	29	29	88	88
Total	$ 160	$ 152	$ 448	$ 440