Reportable Segments - Revenue information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information
Net investment income	$ 974	$ 220	$ 1,682	$ 466
Realized capital gains (losses)	287	440	713	278
Consolidated revenues	12,646	10,403	25,097	20,269
Property-Liability
Segment Reporting Information
Property and casualty insurance premiums	10,009	8,863	19,905	17,744
Net investment income	931	178	1,604	380
Realized capital gains (losses)	265	382	669	279
Consolidated revenues	11,526	9,629	22,884	18,822
Property-Liability | Auto
Segment Reporting Information
Property and casualty insurance premiums	6,883	6,172	13,692	12,327
Property-Liability | Homeowners
Segment Reporting Information
Property and casualty insurance premiums	2,411	2,054	4,803	4,091
Property-Liability | Other personal lines
Segment Reporting Information
Property and casualty insurance premiums	519	478	1,024	949
Property-Liability | Commercial lines
Segment Reporting Information
Property and casualty insurance premiums	196	159	386	377
Allstate Protection
Segment Reporting Information
Property and casualty insurance premiums	10,009	8,863	19,905	17,744
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	46	35	87	73
Net investment income	12	11	22	21
Realized capital gains (losses)	6	19	16	(5)
Consolidated revenues	587	476	1,149	906
Protection Services | Protection plans
Segment Reporting Information
Insurance premiums, commissions and fees	279	219	539	425
Protection Services | Roadside assistance
Segment Reporting Information
Insurance premiums, commissions and fees	47	44	94	95
Protection Services | Finance and insurance products
Segment Reporting Information
Insurance premiums, commissions and fees	109	97	213	194
Allstate Health and Benefits
Segment Reporting Information
Net investment income	19	20	38	40
Realized capital gains (losses)	4	11	6	(3)
Consolidated revenues	553	294	1,109	582
Allstate Health and Benefits | Employer voluntary benefits
Segment Reporting Information
Insurance premiums, commissions and fees	255	263	518	545
Allstate Health and Benefits | Group health
Segment Reporting Information
Insurance premiums, commissions and fees	87	0	170	0
Allstate Health and Benefits | Individual accident and health
Segment Reporting Information
Insurance premiums, commissions and fees	105	0	214	0
Operating Segments | Property-Liability
Segment Reporting Information
Insurance premiums, commissions and fees	321	206	706	419
Operating Segments | Protection Services
Segment Reporting Information
Insurance premiums, commissions and fees	88	51	178	103
Operating Segments | Allstate Health and Benefits
Segment Reporting Information
Insurance premiums, commissions and fees	83	0	163	0
Operating Segments | Corporate and Other
Segment Reporting Information
Insurance premiums, commissions and fees	2	0	2	0
Net investment income	12	11	18	25
Realized capital gains (losses)	12	28	22	7
Consolidated revenues	26	39	42	32
Intersegment Eliminations
Segment Reporting Information
Consolidated revenues	$ (46)	$ (35)	$ (87)	$ (73)