Reportable Segments - Revenue information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information
Property and casualty insurance premiums	$ 8,286	$ 7,959
Other revenue	216	210
Net investment income	786	748
Realized capital gains and losses	(134)	134
Life premiums and contract charges	616	593
Total revenues	9,770	9,644
Property-Liability
Segment Reporting Information
Net investment income	337	308
Operating Segments | Property-Liability
Segment Reporting Information
Property and casualty insurance premiums	8,019	7,759
Other revenue	174	167
Net investment income	337	308
Realized capital gains and losses	(95)	135
Total revenues	8,435	8,369
Operating Segments | Property-Liability | Auto
Segment Reporting Information
Property and casualty insurance premiums	5,591	5,388
Operating Segments | Property-Liability | Homeowners
Segment Reporting Information
Property and casualty insurance premiums	1,848	1,815
Operating Segments | Property-Liability | Other personal lines
Segment Reporting Information
Property and casualty insurance premiums	444	431
Operating Segments | Property-Liability | Commercial lines
Segment Reporting Information
Property and casualty insurance premiums	136	125
Operating Segments | Allstate Protection
Segment Reporting Information
Property and casualty insurance premiums	8,019	7,759
Operating Segments | Discontinued Lines and Coverages
Segment Reporting Information
Property and casualty insurance premiums	0	0
Operating Segments | Service Businesses
Segment Reporting Information
Other revenue	16	16
Net investment income	5	3
Realized capital gains and losses	(4)	0
Intersegment premiums and service fees	29	28
Total revenues	313	247
Operating Segments | Service Businesses | Consumer product protection plans
Segment Reporting Information
Insurance commissions and fees	123	59
Operating Segments | Service Businesses | Roadside assistance
Segment Reporting Information
Insurance commissions and fees	64	68
Operating Segments | Service Businesses | Finance and insurance products
Segment Reporting Information
Insurance commissions and fees	80	73
Operating Segments | Allstate Life
Segment Reporting Information
Other revenue	26	27
Net investment income	122	120
Realized capital gains and losses	(3)	1
Total revenues	472	469
Operating Segments | Allstate Life | Traditional life insurance premiums
Segment Reporting Information
Life premiums and contract charges	146	140
Operating Segments | Allstate Life | Interest-sensitive life insurance contract charges
Segment Reporting Information
Life premiums and contract charges	181	181
Operating Segments | Allstate Benefits
Segment Reporting Information
Net investment income	19	17
Realized capital gains and losses	(2)	0
Total revenues	303	286
Operating Segments | Allstate Benefits | Traditional life insurance premiums
Segment Reporting Information
Life premiums and contract charges	9	9
Operating Segments | Allstate Benefits | Accident and health insurance premiums
Segment Reporting Information
Life premiums and contract charges	248	232
Operating Segments | Allstate Benefits | Interest-sensitive life insurance contract charges
Segment Reporting Information
Life premiums and contract charges	29	28
Operating Segments | Allstate Annuities
Segment Reporting Information
Net investment income	290	289
Realized capital gains and losses	(29)	(2)
Total revenues	264	290
Operating Segments | Allstate Annuities | Fixed annuities contract charges
Segment Reporting Information
Life premiums and contract charges	3	3
Operating Segments | Corporate and Other
Segment Reporting Information
Net investment income	13	11
Realized capital gains and losses	(1)	0
Total revenues	12	11
Intersegment Eliminations
Segment Reporting Information
Total revenues	$ (29)	$ (28)