Reporting Segments - Reportable segments revenue information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information
Premiums earned									$ 38,577	$ 36,513	$ 34,678
Net investment income									3,159	3,240	3,401
Realized capital gains and losses									1,885	(877)	445
Total revenues	$ 11,472	$ 11,069	$ 11,144	$ 10,990	$ 9,481	$ 10,465	$ 10,099	$ 9,770	44,675	39,815	39,407
Allstate Protection
Segment Reporting Information
Premiums earned									34,843	32,950	31,433
Discontinued Lines and Coverages
Segment Reporting Information
Premiums earned									0	0	0
Property-Liability
Segment Reporting Information
Premiums earned									34,843	32,950	31,433
Net investment income									1,533	1,464	1,478
Realized capital gains and losses									1,470	(639)	401
Total revenues									38,587	34,513	34,015
Property-Liability | Auto
Segment Reporting Information
Premiums earned									24,188	22,970	21,878
Property-Liability | Homeowners
Segment Reporting Information
Premiums earned									7,912	7,517	7,310
Property-Liability | Other personal lines
Segment Reporting Information
Premiums earned									1,861	1,808	1,750
Property-Liability | Commercial lines
Segment Reporting Information
Premiums earned									882	655	495
Service Businesses
Segment Reporting Information
Intersegment premiums and service fees									154	122	110
Net investment income									42	27	16
Realized capital gains and losses									32	(11)	0
Total revenues									1,649	1,318	1,059
Service Businesses | Consumer product protection plans
Segment Reporting Information
Insurance commissions and fees									633	503	295
Service Businesses | Roadside assistance
Segment Reporting Information
Insurance commissions and fees									238	263	268
Service Businesses | Finance and insurance products
Segment Reporting Information
Insurance commissions and fees									362	332	304
Allstate Life
Segment Reporting Information
Net investment income									514	505	489
Realized capital gains and losses									1	(14)	5
Total revenues									1,983	1,925	1,888
Allstate Life | Traditional life insurance
Segment Reporting Information
Insurance commissions and fees									630	600	568
Allstate Life | Accident and health insurance premiums
Segment Reporting Information
Insurance commissions and fees									2	2	2
Allstate Life | Interest-sensitive life insurance contract charges
Segment Reporting Information
Insurance commissions and fees									711	713	710
Allstate Benefits
Segment Reporting Information
Net investment income									83	77	72
Realized capital gains and losses									12	(9)	1
Total revenues									1,240	1,203	1,157
Allstate Benefits | Traditional life insurance
Segment Reporting Information
Insurance commissions and fees									43	44	42
Allstate Benefits | Accident and health insurance premiums
Segment Reporting Information
Insurance commissions and fees									988	980	928
Allstate Benefits | Interest-sensitive life insurance contract charges
Segment Reporting Information
Insurance commissions and fees									114	111	114
Allstate Annuities
Segment Reporting Information
Net investment income									917	1,096	1,305
Realized capital gains and losses									346	(166)	44
Total revenues									1,276	945	1,363
Allstate Annuities | Fixed annuities contract charges
Segment Reporting Information
Insurance commissions and fees									13	15	14
Corporate and Other
Segment Reporting Information
Net investment income									70	71	41
Realized capital gains and losses									24	(38)	(6)
Total revenues									94	33	35
Operating Segments | Property-Liability
Segment Reporting Information
Insurance commissions and fees									741	738	703
Operating Segments | Service Businesses
Segment Reporting Information
Insurance commissions and fees									188	82	66
Operating Segments | Allstate Life
Segment Reporting Information
Insurance commissions and fees									125	119	114
Intersegment eliminations
Segment Reporting Information
Total revenues									$ (154)	$ (122)	$ (110)