Earnings per Common Share - Computation earnings per common share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Numerator:
Net (loss) income	$ (321)	$ 650
Less: Net loss attributable to noncontrolling interest	(1)	(10)
Net (loss) income attributable to Allstate	(320)	660
Less: Preferred stock dividends	26	26
Net (loss) income applicable to common shareholders, Basic	(346)	634
Net (loss) income applicable to common shareholders, Diluted	$ (346)	$ 634
Denominator:
Weighted average common shares outstanding (in shares)	263.5	278.1
Effect of dilutive potential common shares
Stock options (in shares)	0.0	2.6
Restricted stock units (non-participating) and performance stock awards (in shares)	0.0	1.1
Weighted average common and dilutive potential common shares outstanding (in shares)	263.5	281.8
Earnings per common share:
Earnings per common share - Basic (in dollars per share)	$ (1.31)	$ 2.28
Earnings per common share - Diluted (in dollars per share)	$ (1.31)	$ 2.25
Anti-dilutive options excluded from diluted earnings per common share (in shares)	1.1	1.2
Weighted average dilutive potential common shares excluded due to net loss applicable to common shareholders (in shares)	2.6	0.0