Reportable Segments - Revenue information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information
Net investment income	$ 690	$ 764	$ 1,846	$ 2,446
Net gains (losses) on investments and derivatives	(167)	105	(1,167)	818
Consolidated revenues	13,208	12,480	37,765	37,577
Property-Liability
Segment Reporting Information
Property and casualty insurance premiums	11,157	10,159	32,529	30,064
Net investment income	632	710	1,696	2,314
Net gains (losses) on investments and derivatives	(123)	94	(988)	763
Consolidated revenues	12,030	11,328	34,303	34,212
Property-Liability | Auto
Segment Reporting Information
Property and casualty insurance premiums	7,545	6,912	21,974	20,604
Property-Liability | Homeowners
Segment Reporting Information
Property and casualty insurance premiums	2,776	2,522	8,065	7,325
Property-Liability | Other personal lines
Segment Reporting Information
Property and casualty insurance premiums	540	521	1,616	1,545
Property-Liability | Commercial lines
Segment Reporting Information
Property and casualty insurance premiums	296	204	874	590
Allstate Protection
Segment Reporting Information
Property and casualty insurance premiums	11,157	10,159	32,529	30,064
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	39	46	118	133
Net investment income	13	10	34	32
Net gains (losses) on investments and derivatives	(13)	4	(56)	20
Consolidated revenues	627	601	1,840	1,750
Protection Services | Protection plans
Segment Reporting Information
Insurance premiums, commissions and fees	330	295	961	834
Protection Services | Roadside assistance
Segment Reporting Information
Insurance premiums, commissions and fees	50	50	152	144
Protection Services | Finance and insurance products
Segment Reporting Information
Insurance premiums, commissions and fees	124	111	362	324
Allstate Health and Benefits
Segment Reporting Information
Net investment income	17	18	50	56
Net gains (losses) on investments and derivatives	(6)	(1)	(25)	5
Consolidated revenues	564	562	1,700	1,671
Allstate Health and Benefits | Employer voluntary benefits
Segment Reporting Information
Insurance premiums, commissions and fees	257	251	780	769
Allstate Health and Benefits | Group health
Segment Reporting Information
Insurance premiums, commissions and fees	96	90	285	260
Allstate Health and Benefits | Individual health
Segment Reporting Information
Insurance premiums, commissions and fees	110	119	333	333
Operating Segments | Property-Liability
Segment Reporting Information
Insurance premiums, commissions and fees	364	365	1,066	1,071
Operating Segments | Protection Services
Segment Reporting Information
Insurance premiums, commissions and fees	84	85	269	263
Operating Segments | Allstate Health and Benefits
Segment Reporting Information
Insurance premiums, commissions and fees	90	85	277	248
Operating Segments | Corporate and Other
Segment Reporting Information
Insurance premiums, commissions and fees	23	1	72	3
Net investment income	28	26	66	44
Net gains (losses) on investments and derivatives	(25)	8	(98)	30
Consolidated revenues	26	35	40	77
Intersegment Eliminations
Segment Reporting Information
Consolidated revenues	$ (39)	$ (46)	$ (118)	$ (133)