Earnings per Common Share - Computation earnings per common share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Numerator:
Net (loss) income from continuing operations	$ (683)	$ 206	$ (1,061)	$ 4,047
Less: Net loss attributable to noncontrolling interest	(15)	(7)	(34)	(7)
Net (loss) income from continuing operations attributable to Allstate	(668)	213	(1,027)	4,054
Less: Preferred stock dividends	26	30	79	87
Net (loss) income from continuing operations applicable to common shareholders	(694)	183	(1,106)	3,967
Income (loss) from discontinued operations, net of tax	0	325	0	(3,272)
Net (loss) income applicable to common shareholders	$ (694)	$ 508	$ (1,106)	$ 695
Denominator:
Weighted average common shares outstanding (in shares)	268.7	293.1	273.5	298.1
Effect of dilutive potential common shares
Stock options (in shares)	0.0	3.1	0.0	2.9
Restricted stock units (non-participating) and performance stock awards (in shares)	0.0	1.7	0.0	1.6
Weighted average common and dilutive potential common shares outstanding (in shares)	268.7	297.9	273.5	302.6
Basic
Continuing operations (in dollars per share)	$ (2.58)	$ 0.62	$ (4.04)	$ 13.31
Discontinued operations (in dollars per share)	0	1.11	0	(10.98)
Earnings per common share applicable to common shareholders - Basic (in dollars per share)	(2.58)	1.73	(4.04)	2.33
Diluted
Continuing operations (in dollars per share)	(2.58)	0.62	(4.04)	13.11
Discontinued operations (in dollars per share)	0	1.09	0	(10.81)
Earnings per common share applicable to common shareholders - Diluted (in dollars per share)	$ (2.58)	$ 1.71	$ (4.04)	$ 2.30
Anti-dilutive options excluded from diluted earnings per common share (in shares)	2.4	0.6	1.6	1.3
Weighted average dilutive potential common shares excluded due to net loss applicable to common shareholders (in shares)	2.9	0.0	3.3	0.0