Reportable Segments - Revenue information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information
Net investment income	$ 764	$ 464	$ 2,446	$ 930
Realized capital gains (losses)	105	319	818	597
Consolidated revenues	12,480	10,678	37,577	30,947
Property-Liability
Segment Reporting Information
Property and casualty insurance premiums	10,159	8,952	30,064	26,696
Net investment income	710	422	2,314	802
Realized capital gains (losses)	94	292	763	571
Consolidated revenues	11,328	9,886	34,212	28,708
Property-Liability | Auto
Segment Reporting Information
Property and casualty insurance premiums	6,912	6,210	20,604	18,537
Property-Liability | Homeowners
Segment Reporting Information
Property and casualty insurance premiums	2,522	2,073	7,325	6,164
Property-Liability | Other personal lines
Segment Reporting Information
Property and casualty insurance premiums	521	486	1,545	1,435
Property-Liability | Commercial lines
Segment Reporting Information
Property and casualty insurance premiums	204	183	590	560
Allstate Protection
Segment Reporting Information
Property and casualty insurance premiums	10,159	8,952	30,064	26,696
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	36	133	109
Net investment income	10	12	32	33
Realized capital gains (losses)	4	14	20	9
Consolidated revenues	601	498	1,750	1,404
Protection Services | Protection plans
Segment Reporting Information
Insurance premiums, commissions and fees	295	236	834	661
Protection Services | Roadside assistance
Segment Reporting Information
Insurance premiums, commissions and fees	50	48	144	143
Protection Services | Finance and insurance products
Segment Reporting Information
Insurance premiums, commissions and fees	111	100	324	294
Allstate Health and Benefits
Segment Reporting Information
Net investment income	18	18	56	58
Realized capital gains (losses)	(1)	3	5	0
Consolidated revenues	562	308	1,671	890
Allstate Health and Benefits | Employer voluntary benefits
Segment Reporting Information
Insurance premiums, commissions and fees	251	287	769	832
Allstate Health and Benefits | Group health
Segment Reporting Information
Insurance premiums, commissions and fees	90	0	260	0
Allstate Health and Benefits | Individual accident and health
Segment Reporting Information
Insurance premiums, commissions and fees	119	0	333	0
Operating Segments | Property-Liability
Segment Reporting Information
Insurance premiums, commissions and fees	365	220	1,071	639
Operating Segments | Protection Services
Segment Reporting Information
Insurance premiums, commissions and fees	85	52	263	155
Operating Segments | Allstate Health and Benefits
Segment Reporting Information
Insurance premiums, commissions and fees	85	0	248	0
Operating Segments | Corporate and Other
Segment Reporting Information
Insurance premiums, commissions and fees	1	0	3	0
Net investment income	26	12	44	37
Realized capital gains (losses)	8	10	30	17
Consolidated revenues	35	22	77	54
Intersegment Eliminations
Segment Reporting Information
Consolidated revenues	$ (46)	$ (36)	$ (133)	$ (109)