Reportable Segments - Performance data and tax items (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information
Amortization of DAC	$ 5,630	$ 5,533	$ 5,222
Income tax expense (benefit)	1,383	1,242	468
Property-Liability
Segment Reporting Information
Amortization of DAC	4,642	4,649	4,475
Income tax expense (benefit)	1,382	1,196	613
Protection Services
Segment Reporting Information
Amortization of DAC	658	543	463
Income tax expense (benefit)	26	(18)	(19)
Allstate Life
Segment Reporting Information
Amortization of DAC	149	173	132
Income tax expense (benefit)	17	53	75
Allstate Benefits
Segment Reporting Information
Amortization of DAC	177	161	145
Income tax expense (benefit)	28	35	32
Allstate Annuities
Segment Reporting Information
Amortization of DAC	4	7	7
Income tax expense (benefit)	(7)	73	(66)
Corporate and Other
Segment Reporting Information
Income tax expense (benefit)	$ (63)	$ (97)	$ (167)