Earnings per Common Share - Schedule of Computation of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Numerator:
Net loss	$ (4)	$ (674)	$ (1,700)	$ (1,046)
Less: Net income (loss) attributable to noncontrolling interest	1	(15)	(23)	(34)
Net loss attributable to Allstate	(5)	(659)	(1,677)	(1,012)
Less: Preferred stock dividends	36	26	99	79
Net loss applicable to common shareholders, basic	(41)	(685)	(1,776)	(1,091)
Net loss applicable to common shareholders, diluted	$ (41)	$ (685)	$ (1,776)	$ (1,091)
Denominator:
Weighted average common shares outstanding (in shares)	261.8	268.7	262.6	273.5
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)	261.8	268.7	262.6	273.5
Earnings per common share:
Earnings per common share - Basic (in dollars per share)	$ (0.16)	$ (2.55)	$ (6.76)	$ (3.99)
Earnings per common share - Diluted (in dollars per share)	$ (0.16)	$ (2.55)	$ (6.76)	$ (3.99)
Anti-dilutive options excluded from diluted earnings per common share (in shares)	3.1	2.4	3.0	1.6
Weighted average dilutive potential common shares excluded due to net loss applicable to common shareholders (in shares)	1.5	2.9	1.9	3.3