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, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Numerator:
Net income from continuing operations									$ 5,159	$ 5,429	$ 4,327
Less: Net loss attributable to noncontrolling interest									(33)	0	0
Net income from continuing operations attributable to Allstate									5,192	5,429	4,327
Less: Preferred stock dividends									114	115	169
Net income from continuing operations applicable to common shareholders	$ 1,111	$ 183	$ 1,399	$ 2,385	$ 2,244	$ 1,189	$ 1,080	$ 801	5,078	5,314	4,158
(Loss) income from discontinued operations, net of tax	(321)	325	196	(3,793)	354	(63)	144	(288)	(3,593)	147	520
Net income applicable to common shareholders	$ 790	$ 508	$ 1,595	$ (1,408)	$ 2,598	$ 1,126	$ 1,224	$ 513	$ 1,485	$ 5,461	$ 4,678
Denominator:
Weighted average common shares outstanding (in shares)									294.8	311.6	328.2
Effect of dilutive potential common shares:
Stock options (in shares)									2.7	2.2	3.2
Restricted stock units (non-participating) and performance stock awards (in shares)									1.6	1.7	2.1
Weighted average common and dilutive potential common shares outstanding (in shares)									299.1	315.5	333.5
Basic
Continuing operations (in dollars per share)	$ 3.90	$ 0.62	$ 4.68	$ 7.88	$ 7.38	$ 3.82	$ 3.44	$ 2.52	$ 17.23	$ 17.06	$ 12.67
Discontinued operations (in dollars per share)	(1.13)	1.11	0.66	(12.53)	1.16	(0.20)	0.46	(0.90)	(12.19)	0.47	1.58
Earnings per common share - Basic (in dollars per share)	2.77	1.73	5.34	(4.65)	8.54	3.62	3.90	1.62	5.04	17.53	14.25
Diluted
Continuing operations (in dollars per share)	3.84	0.62	4.61	7.78	7.30	3.78	3.41	2.48	16.98	16.84	12.47
Discontinued operations (in dollars per share)	(1.11)	1.09	0.65	(12.38)	1.15	(0.20)	0.45	(0.89)	(12.02)	0.47	1.56
Earnings per common share - Diluted (in dollars per share)	$ 2.73	$ 1.71	$ 5.26	$ (4.60)	$ 8.45	$ 3.58	$ 3.86	$ 1.59	$ 4.96	$ 17.31	$ 14.03
Other Earnings Per Share Disclosures
Antidilutive stock options, exercise price exceeds market price (in shares)									1.3	2.9	3.7