Reportable Segments - Revenue information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information
Net investment income	$ 421	$ 648
Realized capital gains (losses)	(462)	662
Total revenues	10,076	10,990
Other revenue
Segment Reporting Information
Insurance premiums, commissions and fees	265	250
Property-Liability
Segment Reporting Information
Premiums Earned, Net	8,881	8,507
Net investment income	202	291
Realized capital gains (losses)	(103)	497
Total revenues	9,161	9,471
Property-Liability | Auto
Segment Reporting Information
Premiums Earned, Net	6,154	5,930
Property-Liability | Homeowners
Segment Reporting Information
Premiums Earned, Net	2,038	1,935
Property-Liability | Other personal lines
Segment Reporting Information
Premiums Earned, Net	471	459
Property-Liability | Commercial lines
Segment Reporting Information
Premiums Earned, Net	218	183
Allstate Protection (1)
Segment Reporting Information
Premiums Earned, Net	8,881	8,507
Discontinued Lines and Coverages
Segment Reporting Information
Premiums Earned, Net	0	0
Service Businesses
Segment Reporting Information
Intersegment premiums and service fees	38	33
Net investment income	10	9
Realized capital gains (losses)	(24)	8
Total revenues	430	392
Service Businesses | Consumer product protection plans
Segment Reporting Information
Insurance premiums, commissions and fees	206	145
Service Businesses | Roadside assistance
Segment Reporting Information
Insurance premiums, commissions and fees	51	63
Service Businesses | Finance and insurance products
Segment Reporting Information
Insurance premiums, commissions and fees	97	87
Allstate Life
Segment Reporting Information
Net investment income	128	127
Realized capital gains (losses)	(31)	(5)
Total revenues	462	486
Allstate Life | Traditional life insurance premiums
Segment Reporting Information
Insurance premiums, commissions and fees	153	154
Allstate Life | Interest-sensitive life insurance contract charges
Segment Reporting Information
Insurance premiums, commissions and fees	180	183
Allstate Benefits
Segment Reporting Information
Net investment income	20	19
Realized capital gains (losses)	(14)	4
Total revenues	288	311
Allstate Benefits | Traditional life insurance premiums
Segment Reporting Information
Insurance premiums, commissions and fees	9	9
Allstate Benefits | Accident and health insurance premiums
Segment Reporting Information
Insurance premiums, commissions and fees	244	250
Allstate Benefits | Interest-sensitive life insurance contract charges
Segment Reporting Information
Insurance premiums, commissions and fees	29	29
Allstate Annuities
Segment Reporting Information
Net investment income	47	190
Realized capital gains (losses)	(269)	156
Total revenues	(220)	349
Allstate Annuities | Fixed annuities contract charges
Segment Reporting Information
Insurance premiums, commissions and fees	2	3
Operating Segments | Property-Liability
Segment Reporting Information
Insurance premiums, commissions and fees	181	176
Operating Segments | Service Businesses
Segment Reporting Information
Insurance premiums, commissions and fees	52	47
Operating Segments | Allstate Life
Segment Reporting Information
Insurance premiums, commissions and fees	32	27
Operating Segments | Corporate and Other
Segment Reporting Information
Net investment income	14	12
Realized capital gains (losses)	(21)	2
Total revenues	(7)	14
Intersegment Eliminations
Segment Reporting Information
Total revenues	$ (38)	$ (33)