Reportable Segments - Revenue information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information
Net investment income	$ 409	$ 942	$ 830	$ 1,590
Realized capital gains (losses)	704	324	242	986
Total revenues	11,197	11,144	21,273	22,134
Other revenue
Segment Reporting Information
Insurance premiums, commissions and fees	257	271	522	521
Property-Liability
Segment Reporting Information
Premiums Earned, Net	8,863	8,681	17,744	17,188
Net investment income	178	471	380	762
Realized capital gains (losses)	382	256	279	753
Total revenues	9,605	9,598	18,766	19,069
Property-Liability | Auto
Segment Reporting Information
Premiums Earned, Net	6,173	6,035	12,327	11,965
Property-Liability | Homeowners
Segment Reporting Information
Premiums Earned, Net	2,053	1,958	4,091	3,893
Property-Liability | Other personal lines
Segment Reporting Information
Premiums Earned, Net	478	462	949	921
Property-Liability | Commercial lines
Segment Reporting Information
Premiums Earned, Net	159	226	377	409
Allstate Protection
Segment Reporting Information
Premiums Earned, Net	8,863	8,681	17,744	17,188
Discontinued Lines and Coverages
Segment Reporting Information
Premiums Earned, Net	0	0	0	0
Service Businesses
Segment Reporting Information
Intersegment premiums and service fees	35	33	73	66
Net investment income	11	10	21	19
Realized capital gains (losses)	19	9	(5)	17
Total revenues	476	405	906	797
Service Businesses | Consumer product protection plans
Segment Reporting Information
Insurance premiums, commissions and fees	219	153	425	298
Service Businesses | Roadside assistance
Segment Reporting Information
Insurance premiums, commissions and fees	44	63	95	126
Service Businesses | Finance and insurance products
Segment Reporting Information
Insurance premiums, commissions and fees	97	89	194	176
Allstate Life
Segment Reporting Information
Net investment income	123	125	251	252
Realized capital gains (losses)	19	1	(12)	(4)
Total revenues	505	492	967	978
Allstate Life | Traditional life insurance premiums
Segment Reporting Information
Insurance premiums, commissions and fees	164	156	317	310
Allstate Life | Accident and health insurance premiums
Segment Reporting Information
Insurance premiums, commissions and fees	1	1	1	1
Allstate Life | Interest-sensitive life insurance contract charges
Segment Reporting Information
Insurance premiums, commissions and fees	174	176	354	359
Allstate Benefits
Segment Reporting Information
Net investment income	20	21	40	40
Realized capital gains (losses)	11	2	(3)	6
Total revenues	294	307	582	618
Allstate Benefits | Traditional life insurance premiums
Segment Reporting Information
Insurance premiums, commissions and fees	12	10	21	19
Allstate Benefits | Accident and health insurance premiums
Segment Reporting Information
Insurance premiums, commissions and fees	225	246	469	496
Allstate Benefits | Interest-sensitive life insurance contract charges
Segment Reporting Information
Insurance premiums, commissions and fees	26	28	55	57
Allstate Annuities
Segment Reporting Information
Net investment income	66	296	113	486
Realized capital gains (losses)	245	48	(24)	204
Total revenues	313	348	93	697
Allstate Annuities | Fixed annuities contract charges
Segment Reporting Information
Insurance premiums, commissions and fees	2	4	4	7
Operating Segments | Property-Liability
Segment Reporting Information
Insurance premiums, commissions and fees	182	190	363	366
Operating Segments | Service Businesses
Segment Reporting Information
Insurance premiums, commissions and fees	51	48	103	95
Operating Segments | Allstate Life
Segment Reporting Information
Insurance premiums, commissions and fees	24	33	56	60
Operating Segments | Corporate and Other
Segment Reporting Information
Net investment income	11	19	25	31
Realized capital gains (losses)	28	8	7	10
Total revenues	39	27	32	41
Intersegment Eliminations
Segment Reporting Information
Total revenues	$ (35)	$ (33)	$ (73)	$ (66)