Earnings per Common Share - Computation earnings per common share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Numerator:
Net (loss) income from continuing operations	$ (1,024)	$ 1,435	$ (378)	$ 3,841
Less: Net (loss) income attributable to noncontrolling interest	(9)	6	(19)	0
Net (loss) income from continuing operations attributable to Allstate	(1,015)	1,429	(359)	3,841
Less: Preferred stock dividends	27	30	53	57
Net (loss) income from continuing operations applicable to common shareholders	(1,042)	1,399	(412)	3,784
Income (loss) from discontinued operations	0	196	0	(3,597)
Net (loss) income applicable to common shareholders	$ (1,042)	$ 1,595	$ (412)	$ 187
Denominator:
Weighted average common shares outstanding (in shares)	273.8	298.8	275.9	300.6
Effect of dilutive potential common shares:
Stock options (in shares)	0.0	3.2	0.0	2.8
Restricted stock units (non-participating) and performance stock awards (in shares)	0.0	1.3	0.0	1.5
Weighted average common and dilutive potential common shares outstanding (in shares)	273.8	303.3	275.9	304.9
Basic
Continuing operations (in dollars per share)	$ (3.81)	$ 4.68	$ (1.49)	$ 12.59
Discontinued operations (in dollars per share)	0	0.66	0	(11.97)
Earnings per common share applicable to common shareholders - Basic (in dollars per share)	(3.81)	5.34	(1.49)	0.62
Diluted
Continuing operations (in dollars per share)	(3.81)	4.61	(1.49)	12.41
Discontinued operations (in dollars per share)	0	0.65	0	(11.80)
Earnings per common share applicable to common shareholders - Diluted (in dollars per share)	$ (3.81)	$ 5.26	$ (1.49)	$ 0.61
Anti-dilutive options excluded from diluted earnings per common share (in shares)	1.3	0.8	1.5	2.0
Weighted average dilutive potential common shares excluded due to net loss applicable to common shareholders (in shares)	3.2	0.0	3.5	0.0