Summary of Significant Accounting Policies - Schedule of Computation of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Numerator:
Net (loss) income from continuing operations									$ (213)	$ (1,342)	$ 5,174
Less: Net loss attributable to noncontrolling interest									(25)	(53)	(33)
Net (loss) income from continuing operations attributable to Allstate									(188)	(1,289)	5,207
Less: Preferred stock dividends									128	105	114
Net (loss) income from continuing operations applicable to common shareholders									(316)	(1,394)	5,093
Income (loss) from discontinued operations, net of tax									0	0	(3,593)
Net (loss) income applicable to common shareholders, basic	$ 1,460	$ (41)	$ (1,389)	$ (346)	$ (303)	$ (685)	$ (1,040)	$ 634	(316)	(1,394)	1,500
Net (loss) income applicable to common shareholders, diluted	$ 1,460	$ (41)	$ (1,389)	$ (346)	$ (303)	$ (685)	$ (1,040)	$ 634	$ (316)	$ (1,394)	$ 1,500
Denominator:
Weighted average common shares outstanding (in shares)									262.5	271.2	294.8
Effect of dilutive potential common shares
Stock options (in shares)									0.0	0.0	2.7
Restricted stock units (non-participating) and performance stock awards (in shares)									0.0	0.0	1.6
Weighted average common and dilutive potential common shares outstanding (in shares)									262.5	271.2	299.1
Basic
Continuing operations (in dollars per share)									$ (1.20)	$ (5.14)	$ 17.28
Discontinued operations (in dollars per share)									0	0	(12.19)
Total (in dollars per share)	$ 5.57	$ (0.16)	$ (5.29)	$ (1.31)	$ (1.15)	$ (2.55)	$ (3.80)	$ 2.28	(1.20)	(5.14)	5.09
Diluted
Continuing operations (in dollars per share)									(1.20)	(5.14)	17.03
Discontinued operations (in dollars per share)									0	0	(12.02)
Total (in dollars per share)	$ 5.52	$ (0.16)	$ (5.29)	$ (1.31)	$ (1.15)	$ (2.55)	$ (3.80)	$ 2.25	$ (1.20)	$ (5.14)	$ 5.01
Other Earnings Per Share Disclosures
Anti-dilutive options excluded from diluted earnings per common share (in shares)									3.0	1.7	1.3
Weighted average dilutive potential common shares excluded due to net loss applicable to common shareholders (in shares)									2.2	3.1	0.0