Reportable Segments - Revenue information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information
Net investment income	$ 942	$ 824	$ 1,590	$ 1,610
Realized capital gains and losses	324	(25)	986	(159)
Total revenues	11,144	10,099	22,134	19,869
Other revenue
Segment Reporting Information
Other revenue	271	228	521	444
Property-Liability
Segment Reporting Information
Net investment income	471	353	762	690
Operating Segments | Property-Liability
Segment Reporting Information
Other revenue	8,681	8,189	17,188	16,208
Net investment income	471	353	762	690
Realized capital gains and losses	256	(15)	753	(110)
Total revenues	9,598	8,711	19,069	17,146
Operating Segments | Property-Liability | Auto
Segment Reporting Information
Other revenue	6,035	5,705	11,965	11,296
Operating Segments | Property-Liability | Homeowners
Segment Reporting Information
Other revenue	1,958	1,864	3,893	3,712
Operating Segments | Property-Liability | Other personal lines
Segment Reporting Information
Other revenue	462	455	921	899
Operating Segments | Property-Liability | Commercial lines
Segment Reporting Information
Other revenue	226	165	409	301
Operating Segments | Property-Liability | Other revenue
Segment Reporting Information
Other revenue	190	184	366	358
Operating Segments | Allstate Protection
Segment Reporting Information
Other revenue	8,681	8,189	17,188	16,208
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	33	29	66	58
Net investment income	10	6	19	11
Realized capital gains and losses	9	(2)	17	(6)
Total revenues	405	320	797	633
Operating Segments | Service Businesses | Other revenue
Segment Reporting Information
Other revenue	48	16	95	32
Operating Segments | Service Businesses | Consumer product protection plans
Segment Reporting Information
Other revenue	153	121	298	244
Operating Segments | Service Businesses | Roadside assistance
Segment Reporting Information
Other revenue	63	68	126	132
Operating Segments | Service Businesses | Finance and insurance products
Segment Reporting Information
Other revenue	89	82	176	162
Operating Segments | Allstate Life
Segment Reporting Information
Net investment income	125	130	252	252
Realized capital gains and losses	1	(3)	(4)	(6)
Total revenues	492	481	978	953
Operating Segments | Allstate Life | Other revenue
Segment Reporting Information
Other revenue	33	28	60	54
Operating Segments | Allstate Life | Traditional life insurance premiums
Segment Reporting Information
Other revenue	156	148	310	294
Operating Segments | Allstate Life | Accident and health insurance premiums
Segment Reporting Information
Other revenue	1	1	1	1
Operating Segments | Allstate Life | Interest-sensitive life insurance contract charges
Segment Reporting Information
Other revenue	176	177	359	358
Operating Segments | Allstate Benefits
Segment Reporting Information
Net investment income	21	19	40	38
Realized capital gains and losses	2	0	6	(2)
Total revenues	307	302	618	605
Operating Segments | Allstate Benefits | Traditional life insurance premiums
Segment Reporting Information
Other revenue	10	10	19	19
Operating Segments | Allstate Benefits | Accident and health insurance premiums
Segment Reporting Information
Other revenue	246	245	496	493
Operating Segments | Allstate Benefits | Interest-sensitive life insurance contract charges
Segment Reporting Information
Other revenue	28	28	57	57
Operating Segments | Allstate Annuities
Segment Reporting Information
Net investment income	296	293	486	583
Realized capital gains and losses	48	6	204	(23)
Total revenues	348	302	697	566
Operating Segments | Allstate Annuities | Fixed annuities contract charges
Segment Reporting Information
Other revenue	4	3	7	6
Operating Segments | Corporate and Other
Segment Reporting Information
Net investment income	19	23	31	36
Realized capital gains and losses	8	(11)	10	(12)
Total revenues	27	12	41	24
Intersegment Eliminations
Segment Reporting Information
Total revenues	$ (33)	$ (29)	$ (66)	$ (58)