Reportable Segments - Schedule of Reportable Segments Expense Information Used in Measure for Segment Profit or Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information
Prior year reserve reestimates	$ (251)	$ (151)
Accident, health and other policy benefits	333	296
Amortization of DAC	2,087	1,939
Amortization of purchased intangibles	59	69
Interest expense	100	97
Restructuring and related charges	16	10
Income taxes on operations	123	266
Preferred stock dividends	29	29
Non-catastrophe losses
Segment Reporting Information
Prior year reserve reestimates	(235)	11
Catastrophe
Segment Reporting Information
Prior year reserve reestimates	(16)	(162)
Loss adjustment expenses, net of reinsurance	2,200	731
Operating segments | Property-Liability
Segment Reporting Information
Total	14,155	12,432
Loss adjustment expenses, net of reinsurance	734	696
Operating segments | Property-Liability | Allstate Protection
Segment Reporting Information
Property and casualty insurance claims and claims expense	8,693	8,607
Amortization of DAC	1,732	1,608
Advertising expense	523	283
Amortization of purchased intangibles	46	51
Restructuring and related charges	16	7
Other segment expenses	1,177	1,133
Total	14,151	12,427
Operating segments | Property-Liability | Allstate Protection | Non-catastrophe losses
Segment Reporting Information
Prior year reserve reestimates	(238)	7
Operating segments | Property-Liability | Allstate Protection | Catastrophe
Segment Reporting Information
Property and casualty insurance claims and claims expense	2,202	731
Operating segments | Property-Liability | Run-off Property-Liability
Segment Reporting Information
Property and casualty insurance claims and claims expense	3	4
Other segment expenses	1	1
Total	4	5
Operating segments | Protection Services
Segment Reporting Information
Property and casualty insurance claims and claims expense	161	158
Amortization of DAC	318	289
Restructuring and related charges	0	1
Other segment expenses	309	234
Income taxes on operations	17	17
Total	805	699
Operating segments | Allstate Health and Benefits
Segment Reporting Information
Accident, health and other policy benefits	333	296
Amortization of DAC	37	42
Restructuring and related charges	0	1
Other segment expenses	234	225
Income taxes on operations	9	15
Total	613	579
Operating segments | Corporate and Other
Segment Reporting Information
Interest expense	100	97
Restructuring and related charges	0	1
Other segment expenses	32	42
Income taxes on operations	(27)	(25)
Preferred stock dividends	29	29
Total	$ 134	$ 144