Reportable Segments - Revenue information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information
Net investment income	$ 562	$ 974	$ 1,156	$ 1,682
Net gains (losses) on investments and derivatives	(733)	287	(1,000)	713
Consolidated revenues	12,220	12,646	24,557	25,097
Property-Liability
Segment Reporting Information
Property and casualty insurance premiums	10,874	10,009	21,372	19,905
Net investment income	506	931	1,064	1,604
Net gains (losses) on investments and derivatives	(662)	265	(865)	669
Consolidated revenues	11,073	11,526	22,273	22,884
Property-Liability | Auto
Segment Reporting Information
Property and casualty insurance premiums	7,348	6,883	14,429	13,692
Property-Liability | Homeowners
Segment Reporting Information
Property and casualty insurance premiums	2,686	2,411	5,289	4,803
Property-Liability | Other personal lines
Segment Reporting Information
Property and casualty insurance premiums	545	519	1,076	1,024
Property-Liability | Commercial lines
Segment Reporting Information
Property and casualty insurance premiums	295	196	578	386
Allstate Protection
Segment Reporting Information
Property and casualty insurance premiums	10,874	10,009	21,372	19,905
Run-off Property-Liability
Segment Reporting Information
Property and casualty insurance premiums	0	0	0	0
Protection Services
Segment Reporting Information
Intersegment premiums and service fees	38	46	79	87
Net investment income	12	12	21	22
Net gains (losses) on investments and derivatives	(30)	6	(43)	16
Consolidated revenues	599	587	1,213	1,149
Protection Services | Protection plans
Segment Reporting Information
Insurance premiums, commissions and fees	318	279	631	539
Protection Services | Roadside assistance
Segment Reporting Information
Insurance premiums, commissions and fees	49	47	102	94
Protection Services | Finance and insurance products
Segment Reporting Information
Insurance premiums, commissions and fees	121	109	238	213
Allstate Health and Benefits
Segment Reporting Information
Net investment income	16	19	33	38
Net gains (losses) on investments and derivatives	(12)	4	(19)	6
Consolidated revenues	562	553	1,136	1,109
Allstate Health and Benefits | Employer voluntary benefits
Segment Reporting Information
Insurance premiums, commissions and fees	257	255	523	518
Allstate Health and Benefits | Group health
Segment Reporting Information
Insurance premiums, commissions and fees	95	87	189	170
Allstate Health and Benefits | Individual health
Segment Reporting Information
Insurance premiums, commissions and fees	114	105	223	214
Operating Segments | Property-Liability
Segment Reporting Information
Insurance premiums, commissions and fees	355	321	702	706
Operating Segments | Protection Services
Segment Reporting Information
Insurance premiums, commissions and fees	91	88	185	178
Operating Segments | Allstate Health and Benefits
Segment Reporting Information
Insurance premiums, commissions and fees	92	83	187	163
Operating Segments | Corporate and Other
Segment Reporting Information
Insurance premiums, commissions and fees	25	2	49	2
Net investment income	28	12	38	18
Net gains (losses) on investments and derivatives	(29)	12	(73)	22
Consolidated revenues	24	26	14	42
Intersegment Eliminations
Segment Reporting Information
Consolidated revenues	$ (38)	$ (46)	$ (79)	$ (87)