Reportable Segments - Revenue information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information
Net investment income	$ 648	$ 786
Realized capital gains and losses	662	(134)
Total revenues	10,990	9,770
Other revenue
Segment Reporting Information
Other revenue	250	216
Property-Liability
Segment Reporting Information
Net investment income	291	337
Operating Segments | Property-Liability
Segment Reporting Information
Other revenue	8,507	8,019
Net investment income	291	337
Realized capital gains and losses	497	(95)
Total revenues	9,471	8,435
Operating Segments | Property-Liability | Auto
Segment Reporting Information
Other revenue	5,930	5,591
Operating Segments | Property-Liability | Homeowners
Segment Reporting Information
Other revenue	1,935	1,848
Operating Segments | Property-Liability | Other personal lines
Segment Reporting Information
Other revenue	459	444
Operating Segments | Property-Liability | Commercial lines
Segment Reporting Information
Other revenue	183	136
Operating Segments | Property-Liability | Other revenue
Segment Reporting Information
Other revenue	176	174
Operating Segments | Allstate Protection
Segment Reporting Information
Other revenue	8,507	8,019
Operating Segments | Discontinued Lines and Coverages
Segment Reporting Information
Other revenue	0	0
Operating Segments | Service Businesses
Segment Reporting Information
Intersegment premiums and service fees	33	29
Net investment income	9	5
Realized capital gains and losses	8	(4)
Total revenues	392	313
Operating Segments | Service Businesses | Other revenue
Segment Reporting Information
Other revenue	47	16
Operating Segments | Service Businesses | Consumer product protection plans
Segment Reporting Information
Other revenue	145	123
Operating Segments | Service Businesses | Roadside assistance
Segment Reporting Information
Other revenue	63	64
Operating Segments | Service Businesses | Finance and insurance products
Segment Reporting Information
Other revenue	87	80
Operating Segments | Allstate Life
Segment Reporting Information
Net investment income	127	122
Realized capital gains and losses	(5)	(3)
Total revenues	486	472
Operating Segments | Allstate Life | Other revenue
Segment Reporting Information
Other revenue	27	26
Operating Segments | Allstate Life | Traditional life insurance premiums
Segment Reporting Information
Other revenue	154	146
Operating Segments | Allstate Life | Interest-sensitive life insurance contract charges
Segment Reporting Information
Other revenue	183	181
Operating Segments | Allstate Benefits
Segment Reporting Information
Net investment income	19	19
Realized capital gains and losses	4	(2)
Total revenues	311	303
Operating Segments | Allstate Benefits | Traditional life insurance premiums
Segment Reporting Information
Other revenue	9	9
Operating Segments | Allstate Benefits | Accident and health insurance premiums
Segment Reporting Information
Other revenue	250	248
Operating Segments | Allstate Benefits | Interest-sensitive life insurance contract charges
Segment Reporting Information
Other revenue	29	29
Operating Segments | Allstate Annuities
Segment Reporting Information
Net investment income	190	290
Realized capital gains and losses	156	(29)
Total revenues	349	264
Operating Segments | Allstate Annuities | Fixed annuities contract charges
Segment Reporting Information
Other revenue	3	3
Operating Segments | Corporate and Other
Segment Reporting Information
Net investment income	12	13
Realized capital gains and losses	2	(1)
Total revenues	14	12
Intersegment Eliminations
Segment Reporting Information
Total revenues	$ (33)	$ (29)