Reporting Segments - Impacts of Tax Legislation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information
Income tax expense (benefit) before Tax Legislation	$ 1,308
Tax Legislation expense (benefit)	(506)
Total income tax expense	802	$ 877	$ 1,111
Property-Liability
Segment Reporting Information
Income tax expense (benefit) before Tax Legislation	1,253
Tax Legislation expense (benefit)	65
Total income tax expense	1,318	806	867
Service Businesses
Segment Reporting Information
Income tax expense (benefit) before Tax Legislation	(59)
Tax Legislation expense (benefit)	(134)
Total income tax expense	(193)	0	2
Allstate Life
Segment Reporting Information
Income tax expense (benefit) before Tax Legislation	108
Tax Legislation expense (benefit)	(332)
Total income tax expense	(224)	91	108
Allstate Benefits
Segment Reporting Information
Income tax expense (benefit) before Tax Legislation	52
Tax Legislation expense (benefit)	(51)
Total income tax expense	1	51	55
Allstate Annuities
Segment Reporting Information
Income tax expense (benefit) before Tax Legislation	124
Tax Legislation expense (benefit)	(182)
Total income tax expense	(58)	36	188
Corporate and Other
Segment Reporting Information
Income tax expense (benefit) before Tax Legislation	(170)
Tax Legislation expense (benefit)	128
Total income tax expense	$ (42)	$ (107)	$ (109)