Earnings per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Numerator:
Net income	$ 596	$ 1,198
Less: Net income (loss) attributable to noncontrolling interest	1	(20)
Net income attributable to Allstate	595	1,218
Less: Preferred stock dividends	29	29
Net income applicable to common shareholders, basic	566	1,189
Net income applicable to common shareholders, diluted	$ 566	$ 1,189
Denominator:
Weighted average common shares outstanding (in shares)	265.3	263.5
Effect of dilutive potential common shares
Stock options (in shares)	2.6	2.5
Restricted stock units (non-participating) and performance stock awards (in shares)	0.9	0.5
Weighted average common and dilutive potential common shares outstanding (in shares)	268.8	266.5
Earnings per common share:
Earnings per common share - Basic (in dollars per share)	$ 2.13	$ 4.51
Earnings per common share - Diluted (in dollars per share)	$ 2.11	$ 4.46
Anti-dilutive options excluded from diluted earnings per common share (in shares)	0.2	0.7