Reportable Segments - Reportable segments revenue information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information
Insurance premiums									$ 39,517	$ 38,577	$ 36,513
Net investment income									2,853	3,159	3,240
Realized capital gains (losses)	$ 4								1,356	1,885	(877)
Total revenues	$ 12,018	$ 11,500	$ 11,197	$ 10,076	$ 11,472	$ 11,069	$ 11,144	$ 10,990	44,791	44,675	39,815
Allstate Protection
Segment Reporting Information
Insurance premiums									35,580	34,843	32,950
Discontinued Lines and Coverages
Segment Reporting Information
Insurance premiums									0	0	0
Property-Liability
Segment Reporting Information
Insurance premiums									35,580	34,843	32,950
Net investment income									1,421	1,533	1,464
Realized capital gains (losses)									990	1,470	(639)
Total revenues									38,727	38,587	34,513
Property-Liability | Auto
Segment Reporting Information
Insurance premiums									24,640	24,188	22,970
Property-Liability | Homeowners
Segment Reporting Information
Insurance premiums									8,254	7,912	7,517
Property-Liability | Other personal lines
Segment Reporting Information
Insurance premiums									1,919	1,861	1,808
Property-Liability | Commercial lines
Segment Reporting Information
Insurance premiums									767	882	655
Protection Services
Segment Reporting Information
Intersegment premiums and service fees									147	154	122
Net investment income									44	42	27
Realized capital gains (losses)									30	32	(11)
Total revenues									1,922	1,649	1,318
Protection Services | Consumer product protection plans
Segment Reporting Information
Insurance premiums, commissions and fees									909	633	503
Protection Services | Roadside assistance
Segment Reporting Information
Insurance premiums, commissions and fees									188	238	263
Protection Services | Finance and insurance products
Segment Reporting Information
Insurance premiums, commissions and fees									396	362	332
Allstate Life
Segment Reporting Information
Net investment income									502	514	505
Realized capital gains (losses)									(10)	1	(14)
Total revenues									1,953	1,983	1,925
Allstate Life | Traditional life insurance
Segment Reporting Information
Insurance premiums, commissions and fees									633	630	600
Allstate Life | Accident and health insurance premiums
Segment Reporting Information
Insurance premiums, commissions and fees									2	2	2
Allstate Life | Interest-sensitive life insurance contract charges
Segment Reporting Information
Insurance premiums, commissions and fees									705	711	713
Allstate Benefits
Segment Reporting Information
Net investment income									78	83	77
Realized capital gains (losses)									8	12	(9)
Total revenues									1,180	1,240	1,203
Allstate Benefits | Traditional life insurance
Segment Reporting Information
Insurance premiums, commissions and fees									46	43	44
Allstate Benefits | Accident and health insurance premiums
Segment Reporting Information
Insurance premiums, commissions and fees									926	988	980
Allstate Benefits | Interest-sensitive life insurance contract charges
Segment Reporting Information
Insurance premiums, commissions and fees									122	114	111
Allstate Annuities
Segment Reporting Information
Net investment income									761	917	1,096
Realized capital gains (losses)									279	346	(166)
Total revenues									1,050	1,276	945
Allstate Annuities | Fixed annuities contract charges
Segment Reporting Information
Insurance premiums, commissions and fees									10	13	15
Corporate and Other
Segment Reporting Information
Net investment income									47	70	71
Realized capital gains (losses)									59	24	(38)
Total revenues									106	94	33
Other revenue | Property-Liability
Segment Reporting Information
Insurance premiums, commissions and fees									736	741	738
Other revenue | Protection Services
Segment Reporting Information
Insurance premiums, commissions and fees									208	188	82
Other revenue | Allstate Life
Segment Reporting Information
Insurance premiums, commissions and fees									121	125	119
Intersegment eliminations
Segment Reporting Information
Total revenues									$ (147)	$ (154)	$ (122)