Earnings per Common Share - Computation earnings per common share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Numerator:
Net loss	$ (1,375)	$ (1,022)	$ (1,696)	$ (372)
Less: Net loss attributable to noncontrolling interest	(23)	(9)	(24)	(19)
Net loss attributable to Allstate	(1,352)	(1,013)	(1,672)	(353)
Less: Preferred stock dividends	37	27	63	53
Net loss applicable to common shareholders, Basic	(1,389)	(1,040)	(1,735)	(406)
Net loss applicable to common shareholders, Diluted	$ (1,389)	$ (1,040)	$ (1,735)	$ (406)
Denominator:
Weighted average common shares outstanding (in shares)	262.6	273.8	263.1	275.9
Effect of dilutive potential common shares
Stock options (in shares)	0.0	0.0	0.0	0.0
Restricted stock units (non-participating) and performance stock awards (in shares)	0.0	0.0	0.0	0.0
Weighted average common and dilutive potential common shares outstanding (in shares)	262.6	273.8	263.1	275.9
Earnings per common share:
Earnings per common share - Basic (in dollars per share)	$ (5.29)	$ (3.80)	$ (6.59)	$ (1.47)
Earnings per common share - Diluted (in dollars per share)	$ (5.29)	$ (3.80)	$ (6.59)	$ (1.47)
Anti-dilutive options excluded from diluted earnings per common share (in shares)	3.2	1.3	2.9	1.5
Weighted average dilutive potential common shares excluded due to net loss applicable to common shareholders (in shares)	1.7	3.2	2.1	3.5