Reporting Segments - Impacts of Tax Legislation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information
Total income tax expense	$ 1,242	$ 468	$ 995
Property-Liability
Segment Reporting Information
Total income tax expense	1,196	613	1,285
Service Businesses
Segment Reporting Information
Total income tax expense	(18)	(19)	(194)
Allstate Life
Segment Reporting Information
Total income tax expense	53	75	(226)
Allstate Benefits
Segment Reporting Information
Total income tax expense	35	32	1
Allstate Annuities
Segment Reporting Information
Total income tax expense	73	(66)	(58)
Corporate and Other
Segment Reporting Information
Total income tax expense	$ (97)	$ (167)	$ 187