Reportable Segments - Reportable segments revenue information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information
Insurance premiums									$ 44,039	$ 38,167	$ 37,221
Net investment income									3,293	1,590	1,728
Net gains (losses) on investments and derivatives									1,084	1,087	1,538
Total revenues	$ 13,011	$ 12,480	$ 12,646	$ 12,451	$ 10,962	$ 10,678	$ 10,403	$ 9,866	50,588	41,909	41,541
Property-Liability
Segment Reporting Information
Insurance premiums									40,454	35,580	34,843
Net investment income									3,118	1,421	1,533
Net gains (losses) on investments and derivatives									1,021	990	1,470
Total revenues									46,030	38,848	38,712
Property-Liability | Auto
Segment Reporting Information
Insurance premiums									27,623	24,640	24,188
Property-Liability | Homeowners
Segment Reporting Information
Insurance premiums									9,927	8,254	7,912
Property-Liability | Other personal lines
Segment Reporting Information
Insurance premiums									2,077	1,919	1,861
Property-Liability | Commercial lines
Segment Reporting Information
Insurance premiums									827	767	882
Allstate Protection
Segment Reporting Information
Insurance premiums									40,454	35,580	34,843
Run-off Property-Liability
Segment Reporting Information
Insurance premiums									0	0	0
Protection Services
Segment Reporting Information
Intersegment premiums and service fees									175	147	154
Net investment income									43	44	42
Net gains (losses) on investments and derivatives									25	30	32
Total revenues									2,361	1,922	1,649
Protection Services | Protection Plans
Segment Reporting Information
Other revenue									1,132	909	633
Protection Services | Roadside assistance
Segment Reporting Information
Other revenue									192	188	238
Protection Services | Finance and insurance products
Segment Reporting Information
Other revenue									440	396	362
Allstate Health and Benefits
Segment Reporting Information
Net investment income									74	78	83
Net gains (losses) on investments and derivatives									7	8	12
Total revenues									2,261	1,180	1,240
Allstate Health and Benefits | Employer voluntary benefits
Segment Reporting Information
Other revenue									1,031	1,094	1,145
Allstate Health and Benefits | Group health
Segment Reporting Information
Other revenue									350	0	0
Allstate Health and Benefits | Individual health
Segment Reporting Information
Other revenue									440	0	0
Other revenue | Property-Liability
Segment Reporting Information
Other revenue									1,437	857	866
Other revenue | Protection Services
Segment Reporting Information
Other revenue									354	208	188
Other revenue | Corporate and Other
Segment Reporting Information
Other revenue									22	0	0
Net investment income									58	47	70
Net gains (losses) on investments and derivatives									31	59	24
Total revenues									111	106	94
Other revenue | Allstate Health and Benefits
Segment Reporting Information
Other revenue									359	0	0
Intersegment eliminations
Segment Reporting Information
Total revenues									$ (175)	$ (147)	$ (154)