Reportable Segments - Revenue information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information
Net investment income	$ 844	$ 843	$ 2,454	$ 2,488
Realized capital gains and losses	176	103	17	318
Total revenues	10,465	9,888	30,334	29,345
Other revenue
Segment Reporting Information
Other revenue	238	228	682	664
Property-Liability
Segment Reporting Information
Net investment income	410	368	1,100	1,063
Operating Segments | Property-Liability
Segment Reporting Information
Other revenue	8,320	7,896	24,528	23,462
Net investment income	410	368	1,100	1,063
Realized capital gains and losses	126	82	16	302
Total revenues	9,048	8,531	26,194	25,360
Operating Segments | Property-Liability | Auto
Segment Reporting Information
Other revenue	5,798	5,501	17,094	16,327
Operating Segments | Property-Liability | Homeowners
Segment Reporting Information
Other revenue	1,891	1,832	5,603	5,462
Operating Segments | Property-Liability | Other personal lines
Segment Reporting Information
Other revenue	455	439	1,354	1,306
Operating Segments | Property-Liability | Commercial lines
Segment Reporting Information
Other revenue	176	124	477	367
Operating Segments | Property-Liability | Other revenue
Segment Reporting Information
Other revenue	192	185	550	533
Operating Segments | Allstate Protection
Segment Reporting Information
Other revenue	8,320	7,896	24,528	23,462
Operating Segments | Discontinued Lines and Coverages
Segment Reporting Information
Other revenue	0	0	0	0
Operating Segments | Service Businesses
Segment Reporting Information
Intersegment premiums and service fees	31	26	89	82
Net investment income	7	4	18	11
Realized capital gains and losses	0	0	(6)	0
Total revenues	329	272	962	779
Operating Segments | Service Businesses | Other revenue
Segment Reporting Information
Other revenue	16	17	48	50
Operating Segments | Service Businesses | Consumer product protection plans
Segment Reporting Information
Other revenue	125	78	369	207
Operating Segments | Service Businesses | Roadside assistance
Segment Reporting Information
Other revenue	66	69	198	204
Operating Segments | Service Businesses | Finance and insurance products
Segment Reporting Information
Other revenue	84	78	246	225
Operating Segments | Allstate Life
Segment Reporting Information
Net investment income	128	119	380	362
Realized capital gains and losses	(3)	2	(9)	4
Total revenues	477	463	1,430	1,403
Operating Segments | Allstate Life | Other revenue
Segment Reporting Information
Other revenue	30	26	84	81
Operating Segments | Allstate Life | Traditional life insurance premiums
Segment Reporting Information
Other revenue	149	141	443	420
Operating Segments | Allstate Life | Accident and health insurance premiums
Segment Reporting Information
Other revenue	0	0	1	1
Operating Segments | Allstate Life | Interest-sensitive life insurance contract charges
Segment Reporting Information
Other revenue	173	175	531	535
Operating Segments | Allstate Benefits
Segment Reporting Information
Net investment income	19	18	57	54
Realized capital gains and losses	2	1	0	1
Total revenues	306	292	911	866
Operating Segments | Allstate Benefits | Traditional life insurance premiums
Segment Reporting Information
Other revenue	13	12	32	30
Operating Segments | Allstate Benefits | Accident and health insurance premiums
Segment Reporting Information
Other revenue	246	232	739	696
Operating Segments | Allstate Benefits | Interest-sensitive life insurance contract charges
Segment Reporting Information
Other revenue	26	29	83	85
Operating Segments | Allstate Annuities
Segment Reporting Information
Net investment income	260	324	843	967
Realized capital gains and losses	51	18	28	11
Total revenues	316	346	882	988
Operating Segments | Allstate Annuities | Fixed annuities contract charges
Segment Reporting Information
Other revenue	5	4	11	10
Operating Segments | Corporate and Other
Segment Reporting Information
Net investment income	20	10	56	31
Realized capital gains and losses	0	0	(12)	0
Total revenues	20	10	44	31
Intersegment Eliminations
Segment Reporting Information
Total revenues	$ (31)	$ (26)	$ (89)	$ (82)