Summary of Significant Accounting Policies - Earnings per common share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Numerator:
Net (loss) income from continuing operations									$ (1,364)	$ 5,159	$ 5,429
Less: Net loss attributable to noncontrolling interest									(53)	(33)	0
Net (loss) income from continuing operations attributable to Allstate									(1,311)	5,192	5,429
Less: Preferred stock dividends									105	114	115
Net (loss) income from continuing operations applicable to common shareholders	$ (310)	$ (694)	$ (1,042)	$ 630	$ 1,111	$ 183	$ 1,399	$ 2,385	(1,416)	5,078	5,314
Income (loss) from discontinued operations, net of tax	0	0	0	0	(321)	325	196	(3,793)	0	(3,593)	147
Net (loss) income applicable to common shareholders ,Basic	$ (310)	$ (694)	$ (1,042)	$ 630	$ 790	$ 508	$ 1,595	$ (1,408)	(1,416)	1,485	5,461
Net (loss) income applicable to common shareholders , Dilluted									$ (1,416)	$ 1,485	$ 5,461
Denominator:
Weighted average common shares outstanding (in shares)									271.2	294.8	311.6
Effect of dilutive potential common shares
Stock options (in shares)									0.0	2.7	2.2
Restricted stock units (non-participating) and performance stock awards (in shares)									0.0	1.6	1.7
Weighted average common and dilutive potential common shares outstanding (in shares)									271.2	299.1	315.5
Basic
Continuing operations (in dollars per share)	$ (1.17)	$ (2.58)	$ (3.81)	$ 2.27	$ 3.90	$ 0.62	$ 4.68	$ 7.88	$ (5.22)	$ 17.23	$ 17.06
Discontinued operations (in dollars per share)	0	0	0	0	(1.13)	1.11	0.66	(12.53)	0	(12.19)	0.47
Basic, Total (in dollars per share)	(1.17)	(2.58)	(3.81)	2.27	2.77	1.73	5.34	(4.65)	(5.22)	5.04	17.53
Diluted
Continuing operations (in dollars per share)	(1.17)	(2.58)	(3.81)	2.24	3.84	0.62	4.61	7.78	(5.22)	16.98	16.84
Discontinued operations (in dollars per share)	0	0	0	0	(1.11)	1.09	0.65	(12.38)	0	(12.02)	0.47
Diluted, Total (in dollars per share)	$ (1.17)	$ (2.58)	$ (3.81)	$ 2.24	$ 2.73	$ 1.71	$ 5.26	$ (4.60)	$ (5.22)	$ 4.96	$ 17.31
Other Earnings Per Share Disclosures
Anti-dilutive options excluded from diluted earnings per common share (in shares)									1.7	1.3	2.9
Weighted average dilutive potential common shares excluded due to net loss applicable to common shareholders (in shares)									3.1	0.0	0.0