Reportable Segments - Revenue information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information
Net investment income	$ 594	$ 708
Net gains (losses) on investments and derivatives	(267)	426
Consolidated revenues	12,337	12,451
Property-Liability
Segment Reporting Information
Property and casualty insurance premiums	10,498	9,896
Net investment income	558	673
Net gains (losses) on investments and derivatives	(203)	404
Consolidated revenues	11,200	11,358
Property-Liability | Auto
Segment Reporting Information
Property and casualty insurance premiums	7,081	6,809
Property-Liability | Homeowners
Segment Reporting Information
Property and casualty insurance premiums	2,603	2,392
Property-Liability | Other personal lines
Segment Reporting Information
Property and casualty insurance premiums	531	505
Property-Liability | Commercial lines
Segment Reporting Information
Property and casualty insurance premiums	283	190
Allstate Protection
Segment Reporting Information
Property and casualty insurance premiums	10,498	9,896
Run-off Property-Liability
Segment Reporting Information
Property and casualty insurance premiums	0	0
Protection Services
Segment Reporting Information
Intersegment premiums and service fees	41	41
Net investment income	9	10
Net gains (losses) on investments and derivatives	(13)	10
Consolidated revenues	614	562
Protection Services | Protection plans
Segment Reporting Information
Insurance premiums, commissions and fees	313	260
Protection Services | Roadside assistance
Segment Reporting Information
Insurance premiums, commissions and fees	53	47
Protection Services | Finance and insurance products
Segment Reporting Information
Insurance premiums, commissions and fees	117	104
Allstate Health and Benefits
Segment Reporting Information
Net investment income	17	19
Net gains (losses) on investments and derivatives	(7)	2
Consolidated revenues	574	556
Allstate Health and Benefits | Employer voluntary benefits
Segment Reporting Information
Insurance premiums, commissions and fees	266	263
Allstate Health and Benefits | Group health
Segment Reporting Information
Insurance premiums, commissions and fees	94	83
Allstate Health and Benefits | Individual health
Segment Reporting Information
Insurance premiums, commissions and fees	109	109
Operating Segments | Property-Liability
Segment Reporting Information
Insurance premiums, commissions and fees	347	385
Operating Segments | Protection Services
Segment Reporting Information
Insurance premiums, commissions and fees	94	90
Operating Segments | Allstate Health and Benefits
Segment Reporting Information
Insurance premiums, commissions and fees	95	80
Operating Segments | Corporate and Other
Segment Reporting Information
Insurance premiums, commissions and fees	24	0
Net investment income	10	6
Net gains (losses) on investments and derivatives	(44)	10
Consolidated revenues	(10)	16
Intersegment Eliminations
Segment Reporting Information
Consolidated revenues	$ (41)	$ (41)