Reportable Segments - Revenue information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information
Net investment income	$ 708	$ 246
Realized capital gains (losses)	426	(162)
Consolidated revenues	12,451	9,866
Property-Liability
Segment Reporting Information
Property and casualty insurance premiums	9,896	8,881
Net investment income	673	202
Realized capital gains (losses)	404	(103)
Consolidated revenues	11,358	9,193
Property-Liability | Auto
Segment Reporting Information
Property and casualty insurance premiums	6,809	6,155
Property-Liability | Homeowners
Segment Reporting Information
Property and casualty insurance premiums	2,392	2,037
Property-Liability | Other personal lines
Segment Reporting Information
Property and casualty insurance premiums	505	471
Property-Liability | Commercial lines
Segment Reporting Information
Property and casualty insurance premiums	190	218
Allstate Protection
Segment Reporting Information
Property and casualty insurance premiums	9,896	8,881
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	38
Net investment income	10	10
Realized capital gains (losses)	10	(24)
Consolidated revenues	562	430
Protection Services | Protection plans
Segment Reporting Information
Insurance premiums, commissions and fees	260	206
Protection Services | Roadside assistance
Segment Reporting Information
Insurance premiums, commissions and fees	47	51
Protection Services | Finance and insurance products
Segment Reporting Information
Insurance premiums, commissions and fees	104	97
Allstate Health and Benefits
Segment Reporting Information
Net investment income	19	20
Realized capital gains (losses)	2	(14)
Consolidated revenues	556	288
Allstate Health and Benefits | Employer voluntary benefits
Segment Reporting Information
Insurance premiums, commissions and fees	263	282
Allstate Health and Benefits | Group health
Segment Reporting Information
Insurance premiums, commissions and fees	83	0
Allstate Health and Benefits | Individual accident and health
Segment Reporting Information
Insurance premiums, commissions and fees	109	0
Operating Segments | Property-Liability
Segment Reporting Information
Insurance premiums, commissions and fees	385	213
Operating Segments | Protection Services
Segment Reporting Information
Insurance premiums, commissions and fees	90	52
Operating Segments | Allstate Health and Benefits
Segment Reporting Information
Insurance premiums, commissions and fees	80	0
Operating Segments | Corporate and Other
Segment Reporting Information
Net investment income	6	14
Realized capital gains (losses)	10	(21)
Consolidated revenues	16	(7)
Intersegment Eliminations
Segment Reporting Information
Consolidated revenues	$ (41)	$ (38)