Reportable Segments - Revenue information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information
Net investment income	$ 880	$ 844	$ 2,470	$ 2,454
Realized capital gains and losses	197	176	1,183	17
Total revenues	11,069	10,465	33,203	30,334
Other revenue
Segment Reporting Information
Revenue	273	238	794	682
Property-Liability
Segment Reporting Information
Net investment income	448	410	1,210	1,100
Operating Segments | Property-Liability
Segment Reporting Information
Revenue	8,782	8,320	25,970	24,528
Net investment income	448	410	1,210	1,100
Realized capital gains and losses	163	126	916	16
Total revenues	9,588	9,048	28,657	26,194
Operating Segments | Property-Liability | Auto
Segment Reporting Information
Revenue	6,080	5,798	18,045	17,094
Operating Segments | Property-Liability | Homeowners
Segment Reporting Information
Revenue	1,997	1,891	5,890	5,603
Operating Segments | Property-Liability | Other personal lines
Segment Reporting Information
Revenue	469	455	1,390	1,354
Operating Segments | Property-Liability | Commercial lines
Segment Reporting Information
Revenue	236	176	645	477
Operating Segments | Property-Liability | Other revenue
Segment Reporting Information
Revenue	195	192	561	550
Operating Segments | Allstate Protection
Segment Reporting Information
Revenue	8,782	8,320	25,970	24,528
Operating Segments | Discontinued Lines and Coverages
Segment Reporting Information
Revenue	0	0	0	0
Operating Segments | Service Businesses
Segment Reporting Information
Intersegment premiums and service fees	44	31	110	89
Net investment income	11	7	30	18
Realized capital gains and losses	4	0	21	(6)
Total revenues	418	329	1,215	962
Operating Segments | Service Businesses | Other revenue
Segment Reporting Information
Revenue	47	16	142	48
Operating Segments | Service Businesses | Consumer product protection plans
Segment Reporting Information
Revenue	163	125	461	369
Operating Segments | Service Businesses | Roadside assistance
Segment Reporting Information
Revenue	57	66	183	198
Operating Segments | Service Businesses | Finance and insurance products
Segment Reporting Information
Revenue	92	84	268	246
Operating Segments | Allstate Life
Segment Reporting Information
Net investment income	128	128	380	380
Realized capital gains and losses	5	(3)	1	(9)
Total revenues	495	477	1,473	1,430
Operating Segments | Allstate Life | Other revenue
Segment Reporting Information
Revenue	31	30	91	84
Operating Segments | Allstate Life | Traditional life insurance premiums
Segment Reporting Information
Revenue	155	149	465	443
Operating Segments | Allstate Life | Accident and health insurance premiums
Segment Reporting Information
Revenue	0	0	1	1
Operating Segments | Allstate Life | Interest-sensitive life insurance contract charges
Segment Reporting Information
Revenue	176	173	535	531
Operating Segments | Allstate Benefits
Segment Reporting Information
Net investment income	21	19	61	57
Realized capital gains and losses	2	2	8	0
Total revenues	314	306	932	911
Operating Segments | Allstate Benefits | Traditional life insurance premiums
Segment Reporting Information
Revenue	12	13	31	32
Operating Segments | Allstate Benefits | Accident and health insurance premiums
Segment Reporting Information
Revenue	250	246	746	739
Operating Segments | Allstate Benefits | Interest-sensitive life insurance contract charges
Segment Reporting Information
Revenue	29	26	86	83
Operating Segments | Allstate Annuities
Segment Reporting Information
Net investment income	251	260	737	843
Realized capital gains and losses	20	51	224	28
Total revenues	274	316	971	882
Operating Segments | Allstate Annuities | Fixed annuities contract charges
Segment Reporting Information
Revenue	3	5	10	11
Operating Segments | Corporate and Other
Segment Reporting Information
Net investment income	21	20	52	56
Realized capital gains and losses	3	0	13	(12)
Total revenues	24	20	65	44
Intersegment Eliminations
Segment Reporting Information
Total revenues	$ (44)	$ (31)	$ (110)	$ (89)