Reporting Segments - Reportable segments revenue information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information
Insurance premiums									$ 32,300	$ 31,307	$ 30,309
Life and annuity premiums and contract charges									2,378	2,275	2,158
Net investment income									3,401	3,042	3,156
Realized capital gains and losses									445	(90)	30
Total revenues	$ 9,843	$ 9,660	$ 9,587	$ 9,434	$ 9,278	$ 9,221	$ 9,164	$ 8,871	38,524	36,534	35,653
Allstate Protection
Segment Reporting Information
Insurance premiums									31,433	30,727	29,748
Discontinued Lines and Coverages
Segment Reporting Information
Insurance premiums									0	0	0
Property-Liability
Segment Reporting Information
Insurance premiums									31,433	30,727	29,748
Net investment income									1,478	1,253	1,226
Realized capital gains and losses									401	(6)	(237)
Total revenues									33,312	31,974	30,737
Property-Liability | Auto
Segment Reporting Information
Insurance premiums									21,878	21,264	20,410
Property-Liability | Homeowners
Segment Reporting Information
Insurance premiums									7,310	7,257	7,136
Property-Liability | Other personal lines
Segment Reporting Information
Insurance premiums									1,750	1,700	1,692
Property-Liability | Commercial lines
Segment Reporting Information
Insurance premiums									495	506	510
Service Businesses
Segment Reporting Information
Intersegment premiums and service fees (1)									110	105	42
Net investment income									16	13	11
Total revenues									993	698	614
Service Businesses | Consumer product protection plans
Segment Reporting Information
Insurance commissions and fees									295	0	0
Service Businesses | Roadside assistance
Segment Reporting Information
Insurance commissions and fees									268	310	340
Service Businesses | Finance and insurance products
Segment Reporting Information
Insurance commissions and fees									304	270	221
Allstate Life
Segment Reporting Information
Net investment income									489	482	490
Realized capital gains and losses									5	(38)	2
Total revenues									1,774	1,694	1,715
Allstate Life | Traditional life insurance
Segment Reporting Information
Life and annuity premiums and contract charges									568	533	505
Allstate Life | Accident and health insurance
Segment Reporting Information
Life and annuity premiums and contract charges									2	2	2
Allstate Life | Interest-sensitive life insurance
Segment Reporting Information
Life and annuity premiums and contract charges									710	715	716
Allstate Benefits
Segment Reporting Information
Net investment income									72	71	71
Realized capital gains and losses									1	(5)	1
Total revenues									1,157	1,077	993
Allstate Benefits | Traditional life insurance
Segment Reporting Information
Life and annuity premiums and contract charges									42	40	37
Allstate Benefits | Accident and health insurance
Segment Reporting Information
Life and annuity premiums and contract charges									928	857	778
Allstate Benefits | Interest-sensitive life insurance
Segment Reporting Information
Life and annuity premiums and contract charges									114	114	106
Allstate Annuities
Segment Reporting Information
Net investment income									1,305	1,181	1,323
Realized capital gains and losses									44	(38)	264
Total revenues									1,363	1,157	1,601
Allstate Annuities | Fixed annuities
Segment Reporting Information
Life and annuity premiums and contract charges									14	14	14
Corporate and Other
Segment Reporting Information
Net investment income									41	42	35
Realized capital gains and losses									(6)	(3)	0
Total revenues									35	39	35
Intersegment Eliminations (2)
Segment Reporting Information
Total revenues									$ (110)	$ (105)	$ (42)