Earnings per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Numerator:
Net income (loss)									$ 4,599	$ (213)	$ (1,342)
Less: Net loss attributable to noncontrolling interest									(68)	(25)	(53)
Net income (loss) attributable to Allstate									4,667	(188)	(1,289)
Less: Preferred stock dividends									117	128	105
Net income (loss) applicable to common shareholders, basic	$ 1,899	$ 1,161	$ 301	$ 1,189	$ 1,460	$ (41)	$ (1,389)	$ (346)	4,550	(316)	(1,394)
Net income (loss) applicable to common shareholders, diluted	$ 1,899	$ 1,161	$ 301	$ 1,189	$ 1,460	$ (41)	$ (1,389)	$ (346)	$ 4,550	$ (316)	$ (1,394)
Denominator:
Weighted average common shares outstanding (in shares)									264.3	262.5	271.2
Effect of dilutive potential common shares
Stock options (in shares)									2.6	0.0	0.0
Restricted stock units (non-participating) and performance stock awards (in shares)									0.9	0.0	0.0
Weighted average common and dilutive potential common shares outstanding (in shares)									267.8	262.5	271.2
Earnings per common share:
Net income (loss) applicable to common shareholders per common share - Basic (in dollars per share)	$ 7.16	$ 4.39	$ 1.14	$ 4.51	$ 5.57	$ (0.16)	$ (5.29)	$ (1.31)	$ 17.22	$ (1.20)	$ (5.14)
Net income (loss) applicable to common shareholders per common share - Diluted (in dollars per share)	$ 7.07	$ 4.33	$ 1.13	$ 4.46	$ 5.52	$ (0.16)	$ (5.29)	$ (1.31)	$ 16.99	$ (1.20)	$ (5.14)
Anti-dilutive options excluded from diluted earnings per common share (in shares)									0.5	3.0	1.7
Weighted average dilutive potential common shares excluded due to net loss applicable to common shareholders (in shares)									0.0	2.2	3.1